17 Questions To Study For A Brain Anatomy Quiz In AP Biology

Ready to ace your next brain anatomy quiz? We’ve got you covered! Review our 17 practice questions to improve your understanding of the parts of the brain and the central nervous system. Plus, brush up on our tips for studying smarter, not harder.

Taking AP Biology? Have a brain anatomy quiz coming soon? We’ve got 17 questions to help you study for it, plus some clever tricks and tips for studying smarter, not harder!

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Parts of the Brain

One of the first things you should have to ace a brain anatomy quiz is a thorough grasp of the parts of the brain and each part’s function. Here are some of the questions you might expect:

brain cartoon


Where is the cerebellum located and what does it do?

The cerebellum is the part of the brain situated at the back of the head. It receives sensory information and regulates your motor movements. The cerebellum also controls balance and coordination, helping you to enjoy smooth movements.


Which part of the brain processes visual information?

The occipital lobe lies underneath the occipital bone. It is part of the forebrain (you have two, technically; one at the back of each cortex) and is responsible for processing visual information. Here’s a helpful memory device: the “o” in occipital can remind you of the “o” in optometrist or ophthalmologist.


If a person’s frontal lobe is injured, what functions might he or she lose?

The frontal lobe can be found in the front of the brain, in each cerebral hemisphere. A deep groove called the central sulcus separates it from the parietal lobe, and another groove called the lateral sulcus separates it from the temporal lobe. A part of the frontal lobe known as the precentral gyrus contains the primary motor cortex, which controls specific body parts’ voluntary movements.

The frontal lobe is responsible for reasoning, higher order thinking, and creativity, so if somebody’s frontal lobe is damaged, he or she could have difficulty making decisions and reasoning.


What are the gyrus and sulcus and how do they help the brain?

Gyrus are the ridges on the brain and sulcus are the grooves (also seen as furrows or depressions). Together, their up and down “motion” are responsible for the folded, “spaghetti” appearance of the brain.

They are, in fact, an extremely clever way of making the most of very limited space. The brain is limited to the area inside your cranium, but the folding of the brain tissue allows a much greater surface area for cortical tissue, allowing additional cognitive function even in a relatively small space.

The human brain begins as a smooth surface, but as the embryo develops, the brain begins to form the deep indentations and ridges we see in the adult brain.


What part of the brain controls the primitive parts of our body?

brain illustration

Pons is the Latin word for bridge, and that’s exactly what the pons appears to do in the brain, as its physically connected to the brainstem. Like any good bridge, the pons contains neural pathways to move signals to the medulla, cerebellum, and thalamus.

Many of the nuclei contained inside the pons are responsible for relaying signals, as we’ve already described, but other nuclei play roles in primitive functions that we don’t normally consider being within our control, such as respiration, sleep, bladder control, and others.


What is the corpus callosum?

The corpus callosum sits underneath the cerebral cortex. It’s about 10cm long and is a thick, tough bundle of fibers that connects the cerebral hemispheres (right and left), enabling them to communicate with each other.

It has over 200 million axonal projections, making it the largest white matter structure.


Which part of the brain is the newest from an evolutionary perspective?

The cerebrum is the part of the brain that is outermost. In it, the brain can store memories, call upon senses, and establish self-awareness. High order functioning can also take place here and its known for being larger in musicians and left-handed individuals. It is also considered to be the most recent brain development.

parts of the brain


How many lobes is the brain comprised of, and what are their names and functions?

Inside the brain is found the occipital lobe (see question #2), the frontal lobe (see question #3), the parietal lobe, and the temporal lobe. The parietal lobe sits behind the frontal lobe and above the temporal lobe. It is where the body becomes self-aware and plays an important role in language processing.

The temporal lobe plays a role in the processing of sensory input, helping the brain to translate these inputs into meaning. If, for example, you smell apple pie and think of your grandmother, you have your temporal lobe to thank!


Which part of your brain acts like a supercomputer?

The thalamus is the small organ at the very center of your brain that acts as a supercomputer or switchboard, relaying signals throughout the brain. It is one of the most important parts of the brain and regulates motor signals, sleep, and consciousness.

Closely related to the thalamus is the hypothalamus, which sits just underneath the thalamus and regulates the pituitary gland and homeostasis.


Which part of the brain helps you sneeze?

The medulla oblongata (medulla is Latin for “middle”), and the medulla oblongata is located on the brainstem close to the cerebellum. It is responsible for involuntary or autonomic processes, which include vomiting and sneezing. It also helps with breathing, cardiac functions such as heart rate, and blood pressure.

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The Central Nervous System

nervous system

The central nervous system is another important subject likely to show up on a brain anatomy quiz. The questions below will help you better prepare.


What is the central nervous system (CNS) comprised of?

The brain and the spinal cord make up the CNS, which is protected by the skull and the spine’s vertebral canal. It is the command center of the entire body, regulating all activity and processing all sensory inputs.


What role does the midbrain play in the CNS?

The midbrain controls visual reflexes (including automatic eye movements, such as blinking and focusing). It also contains nuclei that link parts of the body’s motor system, including both cerebral hemispheres.


What is a neurotransmitter?

A neurotransmitter is a chemical that a nerve fiber releases when a nerve impulse arrives. It diffuses across the junction or synapse so that the impulse may pass to the next nerve fiber, muscle fiber, or other structure. Both neurotransmitters and inhibitory neurotransmitters are found in the brain.


What is the difference between dopamine and serotonin?

Dopamine and serotonin are both powerful neurotransmitters. Serotonin impacts your sleep, arousal, hunger, and mood, while dopamine impacts your brain’s pleasure and reward system, your learning and attention, and movement.


What is glutamate and why is it important?

Glutamate is the most abundant neurotransmitter found in the CNS; in fact, it accounts for more than 90% off all the synaptic connections in your brain! Some parts of the brain, including granule cells found in the cerebellum, rely on glutamate almost exclusively. Glutamate also plays a vital role in memory and learning.


Can you name the most common inhibitory neurotransmitter in the brain?

GABA (gamma-Aminobutyric acid) is the most common inhibitory neurotransmitter found in the brain. It is considered inhibitory because it helps to calm or reduce neuron excitability. This means it plays an important role in calming anxiety. It also is responsible for the regulation of muscle tone.


What is the neurotransmitter that triggers our fight or flight response?

The fight or flight response is also called the acute stress response or hyperarousal; it is a physiological reaction that occurs when the brain perceives an imminent threat. Epinephrine (also known as adrenaline) is the neurotransmitter most responsible for this response. It can signal an increase in blood flow to muscles and greater blood flow through the heart, among other things (this is why your heart starts to beat quickly when you’re afraid).

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The Quick Guide to Studying Smarter

If you’re reading this article, you’re already well on your way to preparing for your brain anatomy quiz, but here are a few more tips to help you get the most out of your time studying:

Get Lots of Rest

Sleeping instead of studying sounds counterintuitive, but without sleep, your brain will have a hard time committing what you’ve learned to memory. In fact, one of the best things you can do to prepare for a test or quiz is to get a good night’s sleep the night before!

Use Memory Devices

plugging earphone on a device

We’ve already hinted at a few tricks for helping your brain remember facts (did you notice them in the questions above?), but mnemonic devices and facts set to music help those boring facts stick much better than just rote memorization.

Setting the major parts of the brain to your favorite song, for example, can help pique your brain’s interest and increase emotional arousal, increasing your odds of remembering the information!

Finally, make it real. Drawing the brain, using models of the brain, or reading stories about people who have injured certain parts of the brain are all ways to make abstract concepts seem real–and make you more likely to remember them. Good luck!

Sheep Heart Dissection Lab Report


Sheep Heart Dissection


In this investigation, the external and internal heart structure valves of a sheep’s heart organ were examined and identified by dissection. The heart is a muscular organ that pumps oxygenated blood and nutrients throughout the body. A sheep’s heart has four chambers like most mammals including humans. Two of those chambers are receiving chambers called the right and left atrium. The other two chambers are pumping chambers called the right and left ventricle. A sheep heart dissection can help to identify each of these different areas of the heart.

The efficiency in the cycle of blood depends on the sequential contraction of the atriums and ventricles. Whenever the atriums contract this is called the systolic phase and whenever the ventricles contract this is called the diastolic phase. These contractions ensure the regular flow of blood through the heart. The contractions occur one after another to make a heartbeat. The many heart valves such as the tricuspid and mitral heart valves control the flow of blood from each chamber.

Blood flow through the heart starts when the right atrium takes the blood that flows in through the superior or inferior vena cava. The right atrium then fills with blood and pressure causes the tricuspid valve to open. The blood then goes into the right ventricle where it contracts the blood into the pulmonary arteries. These arteries lead to the lungs where blood is then oxygenated. The oxygenated blood then flows from the lungs to the left atrium through the pulmonary veins. Due to pressure the mitral valve, which leads to the left ventricle, opens up and pushes the blood into the left ventricle. The left ventricle then contracts and forces the blood through the aorta, which provides the rest of the body with blood.


Objectives of a sheep heart dissection in a lab:

  • Describe the appearance of the external and internal structures of the animal’s heart organ
  • Name the structure and function of the animal’s heart organ
  • Understand the anatomy and physiology of a sheep’s heart

What is a Sheep Heart Dissection?


A sheep heart dissection involves cutting into particular areas of a sheep’s heart so that we can see each of the different sections and learn more about what each part of a heart looks and feels like. A sheep’s heart and sheep internal anatomy are very similar to a human, so it gives us an opportunity to learn more about what a human heart might look like on the inside.

By dissecting into a heart, we can see each different section in detail and can learn how each section helps in pumping blood around the body. Following is a full explanation and sheep heart dissection guide so that you can easily and safely complete a sheep heart dissection yourself in a lab setting.


How to do a Heart Dissection




The materials needed in this dissection include sheep’s heart, a dissecting tray, a blunt metal probe, a pair of scissors, a scalpel, and a pair of tweezers. The safety equipment needed for this dissection is safety goggles, lab aprons, and gloves. The procedure must be completed according to the safety elements of the lab manual.


Sheep Heart Dissection Guide


Most diagrams of a heart show the left atrium and ventricle on the right-hand side of the picture. This is to show the heart in a way as if it is facing you. If a human was facing you, then the left-hand side of their heart would be on your right, and this is how diagrams usually portray a heart.


Observing the External Anatomy and Areas of the Heart


  1. Start by identifying the left and right sides of the heart. If you look closely, you will see a diagonal line of blood vessels on one side which divide the heart. The half of the heart that includes all of the apex is the left side. This can be confirmed by gently squeezing each side of the heart. The left side of the heart will feel much firmer than the right. This is due to all of the muscles that are required to pump blood to the entire body. The right side of the heart is less firm and weaker as this side only pumps blood to the lungs.
  2. Place the heart down so that the right side is on your right. Take a moment to examine the darker flaps that are located at the top of the heart. These flaps are named auricles. There should be a large opening at the top of the heart right next to the auricles. This is the opening to the superior vena cava – the area which brings blood from the top half of the body to the right atrium. If you stick a probe into this opening, you should feel it go right through into the right atrium. Slightly down and to the left of the superior vena cava lies another opening. Inserting a probe into this opening will also lead to the inside of the right atrium. This other opening is the inferior vena cava, which brings blood up to the heart from the lower tissues. Another blood vessel will be visible next to the left auricle. This is the pulmonary vein, which brings blood up from the lungs and into the left atrium.
  3. Right in the center of the heart, you will see the largest blood vessel. This is the aorta, which is responsible for taking oxygenated blood from the left ventricle to the rest of the body. The aorta branches out when it leaves the heart into more than one artery so it may have more than one opening on the heart that you are examining. If you look closely at the openings, you will see that they are connected to each other.
  4. To the left side of the back of the aorta, you will find another large vessel. This vessel is the pulmonary artery which is responsible for taking blood from the right ventricle to the lungs.

The Dissecting Process and Observing the Internal Anatomy and Areas of the Heart


  1. Insert your scalpal into superior vena cava and make an incision through the wall of the right-hand side atrium and ventricle. The area that we cut is called the pericardium. This is the sac that surrounds the internal areas of the heart. Pull the two sides apart and you should notice three flaps of membrane. These form the tricuspid valve between the right-hand atrium and ventricle. These are connected to flaps of muscle which are named the papillary muscles. They are connected via tendons called chordae tendinae, and these tendons are what are known as the “heartstrings”. This valve allows blood flow from the atrium into the ventricle and prevents blood from backflowing in the opposite direction.
  2. If you insert your probe into the pulmonary artery, you will see it appear in the right ventricle. Make an incision into this artery and look on the inside of it and you should see three small membrane pockets. These pockets form the pulmonary semilunar valve, which is responsible for preventing blood from flowing back into the right ventricle.
  3. Insert you scalpal into the base of the left auricle of the aorta and continue the incision down the left ventricular wall. Between the left atrium and ventricle, you will find the mitral valve. This will have two flaps of membrane connect via tendons to the papillary muscles.
  4. Insert your probe into the aorta and see where it emerges in the left ventricle. Proceed to make an incision in the aorta and observe the inside for three small membrane pockets. These are the aortic semilunar valve and are responsible for preventing blood from flowing backwards into the left ventricle.



Internal Anatomy and Physiology of the Heart & Blood Flow
sheep heart dissection





1) Trace the path of blood from the right atrium to the aorta. The path of blood starts from the superior or inferior vena cava to the right atrium. Then it goes from there to the right ventricle to the pulmonary arteries. The blood flows to the lungs and comes back to the heart through pulmonary veins to the left atrium. The blood then flows down to the left ventricle. The blood then travels from there to the aorta and leaves the body.


2) Pulmonary circulation carries blood between the heart and the lungs. Systemic circulation carries blood to the rest of the body. In what chambers of the heart does pulmonary circulation begin and end? In what chambers does systemic circulation begin and end? Pulmonary circulation begins in the right ventricle and ends in the left atrium. Systemic circulation begins in the left ventricle and ends in the right atrium.


3) What is the function of the septum separating the left and right ventricles? The septum is sort of like a barrier between the two chambers.


4) What is the function of the mitral and tricuspid valves? These valves control the flow of blood into and out of each chamber in the heart. They also prevent blood from flowing backwards.


5) Why are the walls of the left ventricle thicker than the walls of the right ventricle? The left ventricle has thicker walls because it uses this extra muscle to propel blood to and through the aorta to the rest of the body.


Following the steps in the sheep heart dissection guide will give you the tools and knowledge to write a complete essay on the internal and external anatomy of a sheep’s heart.


Osmosis vs. Diffusion 101: Definitions, Examples, and Practice Problems

Osmosis vs. diffusion is misleading as far as titles go. Both are kinds of passive transport. Passive transport is the gradual movement of molecules from one concentration to another until they are equalized, or at least that’s the shortest definition. Osmosis and diffusion are two ways to accomplish this equilibrium.

Both of these types of passive transport are meant to maintain equilibrium between things like gases, nutrients, water, and some wastes. This is the primary way cells maintain a balance between themselves and extracellular fluids. Both osmosis and diffusion cease once the concentration on both sides of a membrane, like a cell wall, are equalized.

What Exactly is O​​​​smosis?

normal osmosis

Osmosis is the movement of water, and some other liquids, across a semipermeable membrane such as a cell wall. Osmosis doesn’t require extra energy or pressure to occur. It’s one type of passive transport that allows some cells to move nutrients in or wastes out without using the body’s precious energy reserves. Osmosis moves down the concentration gradient.

Osmosis usually happens when water outside, or inside, a cell is more concentrated and helps move nutrients and wastes in and out of the cell. This is a crucial way cells are fed or grow. Osmosis isn’t just about feeding cells and helping them develop. It can occur between two compartments when the water level in one cell is higher, or a concentration of elements is suspended in water outside a cell.

In mammals, osmosis effects the number of nutrients, typically, inside or outside a cell. Through osmosis, cells maintain a steady flow of nutrients into the cavity for repairs or growth. It’s only the primary way cells get rid of wastes. In plants, osmosis is usually the only way water is absorbed from the ground and sent up the plant to feed cells. Osmosis does not work without water.

What isDiffusion?


Diffusion is the movement of particles from an area where the particles are dense to an area where the particles are less think. A great example of it is coffee creamer. At first, the creamer is localized to the spot where you poured it in, but after a few minutes, it invades every other part of your coffee cup. Another good example is muddy water mixing with clean water.

Diffusion typically occurs when gases or liquids are directly mixed in varying concentrations. If a membrane or other divider is removed allowing two vapors or liquids to mingle, diffusion is the result once the gas or liquid levels are balanced again. It is significant to body systems responsible for energy production.

Diffusion helps animals and plants maintain life and produce energy. When you breathe, you are using diffusion to keep oxygen flowing in and out of your body. It also helps regulate heat in animals that lack skin pores and sweat glands like dogs. it is essential to plants during their photosynthesis processes. It helps keep their upper levels watered as well.

Osmosis vs. Diffusion Methods


During Osmosis, water molecules pass freely through any semipermeable membrane. This process is spontaneous in both directions until the water concentration on both sides of the layer are equal. The sole purpose of osmosis in cells is to facilitate the movement of nutrients and wastes from outside to inside cells. It regulates the cells hydration during the process as a byproduct.

Anytime the area around the outside of a cell, or a neighboring cell, has a higher concentration of water, osmosis will spontaneously occur until the concentration of water matches on both sides of the cell wall membrane. The same is true if there is more water inside the cell than outside. Osmosis only occurs in the presence of water.

Osmosis also causes cells to swell or deflate based on the amount of water inside or outside the cell. If more water resides outside the cell wall, the cell loses water and tends to shrink. The opposite occurs if more water is outside the cell walls. If the concentration of water remains the same inside and outside, the cell stays the same size and osmosis does not happen. Osmosis always occurs from the lowest to the highest level.

Diffusion is spontaneous just like osmosis but does not require a membrane to pass through. Particles or molecules spread from high concentration areas to low concentration areas. Diffusion creates entropy because it’s random. There’s no measured transfer; it just happens until everything is mixed well. The mixtures that diffuse do become diluted in the process.

Diffusion follows the Second Law of Thermodynamics because it results in a less concentrated area of energy when it completes. It is the nature of diffusion to introduce randomness and reduce concentrations. It’s the process that allows us to breathe in oxygen and exhale carbon dioxide. The level of oxygen in the air outside our body is higher than it is in our lungs. Diffusion lets us equalize the two.

Osmosis plays a prominent role in the distribution of nutrients and wastes in plants and animals. It helps cells function by supplying them with water and nutrients while removing metabolic wastes from inside the cell. In plants, it takes on additional roles to help the plants get water and nutrients from the soil and move them up the plant.

Diffusion can happen through a semipermeable membrane just like osmosis, but it doesn’t require one to work. While osmosis primarily helps cells move nutrients and wastes around, diffusion helps other particles and molecules such as gases pass through cell walls. Both osmosis and diffusion are necessary to continue life.

The Different Types of Osmosis and Diffusion

types of osmosis

There are only two types of true osmosis, forward osmosis and reverse osmosis. Forward osmosis forces lower concentrated particles to move into higher concentrated areas. This is the primary version of osmosis used to filter things like water in nature. Where regular, or reverse, osmosis tends to push particles around, forward osmosis pulls them in. Forward and reverse osmosis are easy to get confused.

Reverse osmosis works off osmotic pressure. When the concentration of water outside, or inside, a membrane reaches a higher level than its neighbor, osmosis is triggered. If osmosis is possible, it usually prevents diffusion from taking place at the same time. Thus, reverse osmosis can be affected by volumetric and atmospheric pressure to force fluids through a membrane to create a forced filtering process.

There are several different types of diffusion:

  • Self-diffusion: measures how much diffusion will occur even with a chemical is at a neutral state.
  • Reverse diffusion: very similar to forward osmosis but relates to more particles such as gases.
  • Photon diffusion: the movement of light through an object and how the object scatters the light.
  • Momentum diffusion: the spread of liquids, mostly, based on the thickness of the liquid. Thicker liquids create higher momentum diffusion.
  • Gaseous diffusion: mainly used to enrich uranium for nuclear reactors and weapons.
  • Knudsen diffusion: a measure of how a particle reacts to a membrane based on the size of the membrane’s pores and the size of the particle.
  • Facilitated diffusion: the spontaneous movement of molecules through a cell membrane at times when osmosis and other forms of diffusion are inhibited.
  • Electron diffusion: the movement of electrons to create an electric current.
  • Effusion: occurs when a gas is filtered through small holes.
  • Surface diffusion: occurs when a dry, powdery substance falls onto the surface of a liquid.
  • Collective diffusion: the diffusion of large quantities of particles within a substance that aid each other in moving about the material.
  • Osmosis: actually just another form of diffusion.

Examples of Diffusion

example of diffusion

Diffusion happens all around and inside us all the time. If you drink tea or coffee, when you add sugar or creamer to them it diffuses until the whole cup is sweeter or creamier. The aroma from air fresheners or cooking food diffuses in the air and invades every room it can reach in your home. These are great examples of passive diffusion since no energy is needed to accomplish diffusion this way.

Plants and animals use diffusion to breathe. Animals draw air into their lungs where it diffuses with the air already in their lungs. This is how we get oxygen into our lungs, and it’s how we get rid of respiratory wastes like carbon dioxide. Carbon dioxide entering a plant’s stomata or oxygen leaving their stomata is how a plant uses diffusion to breathe.

Examples of Osmosis

example of osmosis

Probably one of the best examples of osmosis is water and nutrients entering a plant’s roots from the soil. Animals use osmosis in a similar way except we absorb nutrients and water throughout our digestive system. Unlike plants, animals eat or drink water and nutrients before they consume them for use by cells to grow and repair themselves.

Some Final Notes

The biggest differences between osmosis and diffusion are how plants and animals use these processes to sustain life. Most kinds of diffusion are similar, and osmosis is technically just another form of diffusion. We use diffusion and osmosis all the time, and most people don’t realize it. It occurs naturally, and it’s manufactured, but it’s necessary for life to exist.

Dihybrid Cross Worksheet: Definition, Examples, Practice & More

Genetics plays a significant role in our understanding of how living organisms come to be as well as bettering our overall knowledge of Biology and cells. Learn more about a dihybrid cross worksheet and the role it plays in genetics. 


Dihybrid Cross Worksheet: Definition, Examples, and Practice


It’s incredible to think that genetics can play a role in how we look, feel, express, and even taste things and it can also play an integral part in what kind of apple grows on a tree, as well as the cells that multiply within us. Genetics is an essential part of understanding all living things and can help us to understand Biology better overall.

Like many aspects of science, genetics is not cut and dry. Often people think they have it all figured out and then become easily confused by another factor. Dihybrid cross is a standard experiment in genetics that students of Biology will study.

We will discuss what it is and help you understand it better, so you can express, explain, and answer any of the questions when your instructor hands you a dihybrid cross worksheet.


What Is A Dihybrid Cross Worksheet?


Dihybrid cross in the “mating experiment between two organisms that are identically hybrid for two characteristics.” What’s a hybrid organism? It’s one that is heterozygous (or monohybrid), which means that it has two different genes (or alleles) at a specific point (this point is often referred to as a locus).

A significant amount of organisms, who can sexually reproduce via the sperm and egg process, have two copies of each gene, which allows them to carry two different alleles. An organism that has parts from two different “true-breeding” lines is often referred to as a hybrid.

While machines or vehicles are not living things, we can easily form a comparison to hybrids; we can also consider this concept when thinking about mixed-breed dogs that have two purebred parents, such as a Puggle or Maltipoo.

The concept and name of the dihybrid cross comes from experimenting with and observing the generations that are produced after two “pure” lines reproduce. A dihybrid cross worksheet allows us to predict how likely an offspring is to inherit a particular single trait.


How to Set Up a Dihybrid Cross Worksheet


A dihybrid cross worksheet will help to predict and determine the genotype of an offspring. It does this by determining all the possible combinations of alleles in the gametes of each of the parents.

As an example, half of the gametes get a dominant S and a dominant Y allele. The other half get a recessive s and a recessive y allele. In this case, both parents are producing 25 percent of each of the following: SY, Sy, sY, and sy.

Since each of the parents, in this case, are producing four different combinations, we must draw a four by four punnett square. We must then list the gametes from one of the parents alone one edge of the punnett square, and the gametes for the other parent along another edge of the punnett square. We will then list in each square, the alleles for the first parent, followed by the addition of the alleles from the second parent. Each combination should contain a dominant-recessive allele. 

The final result will form a diagram of all of the possible combinations of genotypes for the offspring of these two parents.

Try out this method out on a dihybrid cross practice worksheet. 


What is Dominant and Recessive? 


The terms “dominant” and “recessive” refer to the inheritance patterns of certain characteristics. This describes how likely it is for a certain phenotype to be passed on from a parent to their offspring.

Beings that reproduce sexually via the sperm and egg process have two copies of each of their genes. Each of these copies, which are known as alleles, are slightly different and never identical. These differences can affect the rate and variation of proteins that are produced. As proteins affect traits, these differences can affect and produce different phenotypes.

Dominant alleles produce dominant phenotypes and dominant traits in people who have one copy of the allele, which comes from just one parent. In order for a recessive allele to produce a recessive phenotype, the being must have two copies, one from each of the parents. Someone that has a dominant and a recessive allele for a gene will have the dominant phenotype and not the recessive phenotype. This means that they are then considered “carriers” of the recessive allele. This is because the recessive allele is there, however, the recessive phenotype is not.

Dominant and recessive disorders can occur when a person has “broken” genes. This results in a broken code for a protein that doesn’t work properly. Since one regular copy of a gene can mask the effects of a broken gene, many disorders of this type are recessive in their single trait inheritance pattern. However, not all disease alleles are recessive.


Monohybrid Cross Example


A monohybrid cross is defined as a genetic cross mix between individuals who have homozygous genotypes, or genotypes which completely recessive, or completely dominant alleles. This results in opposite phenotypes for a particular genetic characteristic.


Following is an example of a monohybrid cross experiment performed by Gregor Mendel…


Mendel’s Dihybrid Cross Experiment


gregor mendel, dihybrid cross worksheet


Gregor Mendel is well known for his work in the field of genetics, and he performed various genetic experiments, including the dihybrid cross, on pea plants in the late 1800s. When he performed dihybrid crosses on plants, he discovered his Law of Independent Assortment.

You might already be familiar with this law of genetics and that it refers to when two or more characteristics are inherited through reproduction, individual hereditary factors independently assort (during gamete or egg production) and give different traits an equal opportunity to occur together.

Even though Mendel was famous for experimenting on pea plants (mostly because the seeds were cheap and readily available), we can consider the dihybrid cross experiment with every living organism from the food we grow to an expanding family.


Let’s observe how Mendel’s Dihybrid Cross experiment looks.


Crossing The P Generation


pea plant under sunlight, dihybrid cross worksheet


Mendel chose a pea plant that was homozygous and dominant for round (RR) yellow (YY) seeds. He crossed the plant with a pea plant that was homozygous and recessive for wrinkled (rr) green (yy) seeds. Remember, homozygous is a particular gene that has identical alleles.

The notation for crossing the two pea plants is RRYY x rryy. The organisms in this first cross are the parental generation or P generation, which should make sense since they are the “parental” organisms that will be reproducing.

The direct offspring from the P generation (RRYY x rryy cross) is known as the F1 generation. All of the plants from the P generations were heterozygous and had round yellow seeds; the genotype was RrYy.


Crossing The F1 Generation: Dihybrid Cross


The dihybrid cross didn’t occur until Mendel crossed two pea plants from the F1 generation and the notation was RrYy x RrYy. The result of the dihybrid cross gave Mendel the F2 generation and a ratio of 9:3:3:1. Here’s what the ratio means:

  • Nine pea plants with round, yellow seeds
  • Three pea plants with round, green seeds
  • Three pea plants with wrinkled, yellow seeds
  • One pea plant with wrinkled, green seeds

From his findings, Mendel deduced that certain pairs of traits in the P generation sorted independently from one another, from one generation and into the next, and that there is never an equal chance of trait occurrence.


Clarifying The Difference Between A Dihybrid and Monohybrid


Until you get a solid understanding of genetics and cells, dihybrid and monohybrid can be a little confusing, even after we’ve discussed Mendel’s experiment, so let’s clarify the two.

Remember, the dihybrid cross deals with two traits and as the name suggests, the monohybrid centers around a difference in just one trait. The parental organisms are both homozygous for the trait being studied (such as color) but have different alleles for that trait.

One parental organism is homozygous dominant, and the other is homozygous recessive. The F1 generation in a monohybrid is all heterozygous (like the dihybrid cross). F2 generation is typically three-fourths dominant phenotype and one-quarter recessive phenotype.



Applying The Dihybrid Cross Experiment


Mendel’s pea plant dihybrid cross experiment is groundbreaking and helped to form genetics as we know it today, but let’s observe a few other examples…


What Are Some Examples of a Dihybrid Cross Worksheet?


Fruit Flies


fruit flies, dihybrid cross worksheet


If you were studying fruit flies and wanted to use the dihybrid cross experiment on them, where would you begin? Some may say that you should breed the hybrid flies together while others would recommend counting the number of each type of fruit fly you have.

The first step is to establish the lines of homozygotes. If you want your heterozygotes to breed, you have to ensure that the P generation is “true.”

In order to get a line of homozygotes, you would need to breed the lines repeatedly and select the flies that only show one allele for each characteristic in their offspring. It would be a lengthy process, but that’s the only way a dihybrid cross experiment could be successful.


Summer Squash


summer squash, dihybrid cross worksheet


Ready for another example that you might find on a dihybrid cross worksheet? Let’s take a look at this problem.

Find the phenotypic and genotypic ratios for the F1 and F2 generation of summer squash. The summer squash has white fruit color (W), which has dominance over yellow fruit color (w). The disk-shaped fruit (D) has dominance over the sphere-shaped fruit (d).

What results will we have if we cross a squash plant true-breeding for white, disk-shaped fruit with a squash plant true-breeding for yellow, sphere-shaped fruit? Remember, we’re looking for the ratios of F1 and F2 generations.

The P1 geno and phenotypes should be WWDD (white, disk-shaped fruit) x wwdd (yellow, sphere-shaped fruit). To figure out your results, you’ll enter your information into a Punnett Square (you can see how this should look when you click on the genetic cross worksheet that we have listed above).

The results for the F2 generation ratios will form the following:

1:2:2:1:4:2:1:2:1 genotypic ratio (look at the details below)

  • 1/16 will be homozygous dominant for both traits (WWDD)
  • 2/16 will be homozygous dominant for color and heterozygous for shape (WWDd)
  • 2/16 will be heterozygous for color and homozygous dominant for shape (WwDD)
  • 1/16 will be homozygous dominant for color and homozygous recessive for shape (WWdd)
  • 4/16 will be heterozygous for both traits  (WwDd)
  • 2/16 will be heterozygous for color and homozygous recessive gene for shape (Wwdd)
  • 1/16 will be homozygous recessive for color and homozygous dominant for shape (wwDD)
  • 2/16 will be homozygous recessive for color and heterozygous for shape (wwDd)
  • 1/16 will be homozygous recessive for both traits (wwdd)

9:3:3:1 phenotypic ratio (look at the details below)

  • 9/16 will have white, disk-shaped fruit
  • 3/16 will have white, sphere-shaped fruit
  • 3/16 will have yellow, disk-shaped fruit
  • 1/16 will have yellow, sphere-shaped fruit

The Offspring of Made-Up Creatures


Let’s take a look at one more example for variety (and practice).

Imagine a made-up creature that has yellow eyes and green fur. We can assume that both creatures are heterozygous for yellow eyes and green fur, let’s find out the genotype and phenotype of the creature’s offspring; Yellow eyes are E, and green fur is F. The recessive traits are red eyes (ee) and yellow fur (ff). What is the chance that the baby will have red eyes and yellow fur?

First, we need to find the genotype of the parents. Remember that they are heterozygous, which means the genotype is Ee for the eyes and Ff for the fur.

After you form your Punnet Square, you should list every possible combination: E-F, E-f, e-F, e-f. If you’ve filled out your square correctly, there’s a one in 16 chance that the creature’s baby will have red eyes and yellow fur because only one box equals the combination eeff.

As you can see the summer squash problem is a little more complex and time-consuming than Mendel’s pea plant (and the made-up creature is a little bit silly), but with practice and the right information, you can complete any problem on a dihybrid cross worksheet with relative ease and determine the likelihood that certain cells and traits will be produced in an offspring.


Practicing Dihybrid Cross Worksheets


A simple search in Google will bring up many different practice worksheets to help you build upon your skills of creating a dihybrid cross worksheet of your own. Many of these practice worksheets will include a dihybrid cross worksheet answer key so that you can practice yourself and be sure that you are doing them correctly. 

Practicing will ensure that you are ready to answer any questions that your college or university professor may have for you regarding a dihybrid cross worksheet.

Molecular Geometry Chart: Definition, Examples, and Study Guides

How much do you know about molecular geometry definition and the shapes of molecules in chemistry? Join us as we define this subject, go over some examples, and list the different structures you will find in an electron and molecular geometry chart. We have also included some study guides to help you go further.


Molecular Geometry Chart: Definition, Examples, and Study Guides

Molecular geometry is typically taught in college and advanced high school classes. This subject uses geometric models to represent the shape and structure of molecules. It allows scientists to get a precise idea of how the number of atoms and electrons are connected. There are also some rules that help scientists predict which shape a molecule will adopt. There are a huge number of molecules and sequences that can be analyzed. From looking at the number of atoms around a molecule to more difficult structures such as DNA sequence – there are many areas of molecular and electron geometry to explore.


What is a Molecular Geometry Chart?

molecular geometry 3d molecules, molecular geometry chart, molecular geometry definition

Molecular geometry is the science of representing molecules in a three-dimensional manner. A molecular geometry chart is a collection of rules on how molecules and electrons will connect and shape a molecule.

Students and scientists can use these charts to create three-dimensional diagrams that represent molecules. These visual representations are interesting because they help students and scientists predict the shape, polarity, and biological activities of a molecule.

These representations can also help with other concepts, such as:

  • Reactivity.
  • Phase of matter.
  • Color.
  • Magnetism.

Molecular geometry can be applied regardless of how complex a molecule is. Students will typically work with simple models at first before learning how to apply these concepts to create detailed models of more complex molecules.


Common Molecular Structures

Molecules obey certain laws when atoms and electrons connect with each other. Molecules will form specific shapes. It is crucial to familiarize yourself with these common shapes so you can determine the correct one.

These are the shapes and structures you will find in a molecular geometry chart:

  • Linear structures.
  • Bent or angular connections.
  • Pyramidal structures.
  • Tetrahedral molecules.

You are probably already familiar with these structures if you have been studying chemistry for a while. Most students have an understanding of these structures thanks to the visual representations of molecules shown in class or textbooks even though they might not know about the rules of molecular geometry.


Polar And Non-Polar Molecules

Certain molecules can be grouped as either polar or non-polar. Other molecules fall between the two.


The geometry of atoms in some molecules is arranged in such a way that one side has a negative charge and the other side has a positive electrical charge. In this case, this type of molecule is called a polar molecule. This means that it has electrical poles. Molecules that aren’t arranged in this way are called non-polar molecules.


Not every molecule that has polar bonds is a polar molecule. For example, carbon dioxide has two polar bonds (C O). However, the molecular geometry of carbon dioxide is linear. This means that the two bonds cancel each other out, resulting in the molecule being non-polar.


What is electronegativity and why does it vary around a molecular geometry chart? Electronegativity is defined as the measure of the tendency of an atom to attract a bonding pair of electrons.


If two atoms are equally electronegative, then both of those atoms have the same tendency to attract a pair of bonding electrons, and therefore it will be found on average halfway between the two atoms. To get a bond such as this, both atoms would usually be the same atom.


If one of the atoms is slightly more electronegative than the other, this will mean that one of the atoms has a fair share of electron density, and this results in that atom becoming slightly negative. At the same time, the opposing atom will become slightly positive. This is a polar bond.


If one of the atoms is incredibly more electronegative than the other, this will result in the electron pair being dragged right over to that electronegative atom. This means that the other atom loses all control of its electron, and the electronegative atom has complete control over both electrons. This results in ions being formed and is called an ionic bond.


Lewis Theory And Valence-Shell Electron-Pair Repulsion Theory

You can’t gain a thorough understanding of molecular geometry without studying the Lewis theory and the Valence-shell electron-pair repulsion theory.

The Lewis theory is about how valence shell electrons bond with an atom. This approach represents atoms and electrons in a two-dimensional manner. You will often start with this simple visual representation called a Lewis structure to determine the correct three-dimensional structure for a molecule.

You can create a Lewis structure electron dot diagram by simply writing the symbol for the atom you want to represent. You would then add dots to represent the valence shell electrons connected to the atom.

A valence electron is an electron located in the outer shell of the atom. It is common to omit the electrons that aren’t connected to the outer shells of an atom since they won’t form bonds with other elements and would only make Lewis structure diagrams more complicated. If a shell is closed, the electrons that can be found in this shell won’t be shown on a Lewis electron dot diagram.

In a Lewis structure, you can place the dots on any side of the symbol. However, you can’t have more than two dots per side.

The Valence shell electron-pair repulsion theory states that electrons will naturally repel each other. This applies regardless of the type of pairs they form.

Electrons can form bonded pairs, lone pairs, double bonds, triple bonds, or exist as single unpaired electrons. They will position themselves around an atom so that there is as much space as possible between them. In molecular geometry, each pair or single unpaired electron counts as an electron group.

This natural behavior reduces repulsion between electrons and maximizes the space available to attract other elements the electrons can bond with.

The Valence-shell electron-pair repulsion theory is important vital in the context of molecular geometry because it will help you figure out the angle of the different bonds that exist around an atom. You can use this theory to deduce the structure of a molecule.


Molecular Geometry Structures And Atom Groups

You can determine the structure and shape of a molecule once you know about the atoms that make up this molecule, how many electrons are connected to these atoms, and which type of bond these electrons form.

These are the different structures molecules can adopt:

  • If you have an atom with two electron groups, the molecule will adopt take a linear structure.
  • If you have three electron groups, you will have a trigonal-planar structure.
  • The molecule will adopt a bent structure if there are three groups of electrons with a lone pair.
  • If there are four groups of electrons, the molecule will have a tetrahedral shape.
  • A molecule with four groups of electrons and a lone pair of electrons will take the shape of a trigonal pyramid.
  • If you have four groups of electrons and two lone pairs, the molecule will have a bent structure.
  • A molecule with five electron groups will adopt a trigonal-bipyramidal structure.
  • If there are five electron groups and a lone pair of electrons, the molecule will have a seesaw structure.
  • A molecule with five electron groups and two lone pairs, the molecule will be T-shape shaped.
  • If there are five electron groups and three lone pairs, you will have a linear structure.
  • A molecule with six electron groups will have an octahedral shape.
  • A molecule with six electron groups and a lone pair will be shaped like a square pyramid.
  • If you have six electron groups and two lone pairs, the molecule will adopt a square planar structure.

These thirteen different scenarios cover all the different structures you will encounter in molecular geometry. You don’t need to learn them by heart since you can easily deduct the structure that makes the most sense by applying the Valence-shell electron-pair repulsion theory.


Molecules With More Than One Atom

blue and red molecules, molecular geometry chart, molecular geometry definition

The molecular geometry chart still applies if you have a molecule with more than one atom. The structure will be more complex and will probably combine different geometric shapes.

If you have a complex molecule, break it down into smaller section and look at each atom individually. Determine how the electrons will connect to this atom in function of the type of bond they form and of the number of electron groups.

You can use molecular geometry rules to determine the shape and structure of each atom and its electrons. You can then apply the Valence-shell electron-pair repulsion theory to determine how these different small structures will connect to each other to form a more complex molecule.



An ion is defined as a molecule or atom which has acquired an electrical charge due to gaining or losing electrons. Polyatomic ion charges are ones which are composed of two or more atoms. When a polyatomic ion is involved in a chemical reaction, the oxidation number of it plays a significant role. This is a number which is either negative, positive, or zero. This oxidation number is an indication of the number of electrons in which an ion can share, lose, or gain when chemically reacting which an atom, compound, molecule, or with another ion. The oxidation number of a polyatomic ion is worked out at the sum of the oxidation numbers that belong to its constituent atoms. This is equal to the positive charge, negative charge or neutral charge that exists on the ion.


Diatomic Molecules

A diatomic molecule consists of two atoms. The majority of diatomic molecules are ones of the same element, however, a few combine different elements. Most diatomic molecules are gases at room temperature.


Octahedral Molecular Geometry

Octahedral molecular geometry refers to the shape of the compounds that have six atoms or ligands symmetrically arranged around a central atom. This defines the vertices of an octahedral shape. The octahedron is platonic solid, even though octahedral molecules tend to have a central atom and no bonds. The term “octahedral” is a loosely used term amongst scientists, as it focuses purely on the geometry of bonds to the central atom, rather than looking at the differences in the ligands.


Bond Angles And Three-Dimensional Geometry

Students often make the mistake of thinking in two dimensions when determining bond angles. Electrons will position themselves to be as far away from each other as possible, but keep in mind that they will do this on a three-dimensional plane. A molecular geometry chart with bond angles will help to clarify these structures.

Working with three-dimensional models is a great way to get used to thinking about geometry in a three-dimensional plane. There are also apps and software you can use to create virtual models and get used to these concepts.

What is the Difference Between Electron-Pair Geometry and Molecular Structure?

Electron Geometry

Electron geometry is the term used for the geometry of the electron pair located on the central atom. This applies whether they are bonding electrons or non-bonding electrons. The definitions of an electron pair is electrons that are in pairs or multiple bonds, lone pairs and sometimes even just one single electron that is unpaired. Electrons are always in constant motion and it can be difficult to determine the path that they are going to take. With this in mind, the arrangement of electrons within a molecule is defined by electron density distribution.


An Example of Electron Geometry

We will use CH4 as an example.


The central atom in this example is C, plus there are also four valence shell electrons. The hydrogen atoms give four electrons, so that means that there are a total of eight electrons around C. In this example, the single number of bonds are four, and the number of lone pairs here is zero. So with this in mind, we can determine here that the electron geometry of CH4 is tetrahedral.


Molecular Geometry

Molecular geometry is the term used when determining the shape of a molecule. This refers to the three-dimensional structure or arrangement of the atoms within a molecule. There is usually a central atom which is surrounded by electrons. When we understand the molecular geometry of a compound, this makes it much easier to determine the magnetism, phase of matter, polarity, color, and reactivity of that compound. The geometry of molecules is often described using bond lengths, bond angles, and torsional angles. With smaller molecules, simply the molecular formula, along with a table of angles and standard bond lengths may be all that is needed to determine the geometry of that particular molecule. It is predicted by looking at only the electron pairs, which is what makes molecular geometry different from electron geometry.


An Example of Molecular Geometry

We will use H2O as an example.

H2O is a common polar molecule. The central atom in this case is the oxygen atom which has six valence electrons. Hydrogen, in this case, gives a total of two electrons, which makes the total overall amount of electrons eight. In this example, there are two lone electron pairs, and four electron groups, as well as two single bond pairs. Therefore, the molecular geometry in this example is bent.


Why Do Most Atoms Form Chemical Bonds?


Most elements contain atoms that form chemical bonds. This is because those atoms become more stable when they are bonded together. Neighbouring atoms are attracted to each other by electrical forces which make them stick together. Atoms that are strongly attractive rarely spend much time on their own – other atoms will usually bond to them quite quickly. The arrangement of electrons around a central atom is what determines the strength in which it seeks out other atoms to bond with.  


Chemical Bonds

A chemical bond is formed by the joining of two or more atoms. A stable compound occurs when the total energy of the combined atoms has lower energy than if the atoms were separate. The combined state of these atoms implies there is an attractive force between these atoms – a chemical bond. There are two extreme cases of chemical bonds. These are covalent formula bonds and ionic formula bonds.


Covalent Bonding

A covalent formula bond is one which involves sharing of valence electrons by two atoms. These types of covalent bonding can create stable molecules, as long as they share electrons in a way that creates noble gas electron configuration for each atom.


Ionic Bonds

During chemical bonds, atoms can either share or transfer their valence electrons. In some extreme cases, one or more atoms may lose electrons and then other atoms gain them which produces noble gas electron configuration. The bond in this case is called an ionic bond.


What Is The Difference Between Atomic Orbitals And Molecular Orbitals?


The orbital is a region in which the probability of finding an electron is relatively higher than usual. Atoms have a nucleus and within this nucleus they have their own electrons rotating around. When orbitals are overlapped to create molecules via bonding, these types of orbitals are named molecular orbitals. Molecular orbital theory and valence bond theory both explain the properties of molecular and atomic orbitals. Orbitals can hold two electrons within them maximum. The main difference between molecular orbital calculation and atomic orbital calculation is that electrons within a molecular orbital are influenced by two or more nuclei. This depends on the number of atoms in the molecule. Atomic orbitals are different as they are only influenced by one positive nucleus.


Organic Chemistry


Organic chemistry refers to the study of the composition, structure, properties, preparation, and reactions of compounds which contain carbon. This includes compounds that are not only hydrocarbons, but also compounds which contains a number of other elements such as hydrogen, oxygen, halogens, nitrogen, phosphorus, sulfur, and silicon. This type of chemistry used to only refer to compounds that were produced by living beings. However, it has now been broadened to focus on other substances such as plastics. There is a huge range of organic compounds and these include things such as food, explosives, paints, cosmetics, and pharmaceuticals. Organic chemistry is the best way of creating new compounds. Scientists will develop new and better ways of synthesizing existing compounds with organic chemistry.



Study Guides

There are several study guides on molecular geometry chart information which you can use to go further and practice molecular geometry. This subject gets easier once you start applying it and become familiar with all the different structures molecules can adopt.

You should start by modeling simple molecules or look at three-dimensional models of different molecules to identify their shape and ask yourself what kind of atom groups and bond types caused the molecule to adopt this structure.

Here are a few study guides to help you go further:

  • This study guide from NYU covers the basics and includes some helpful diagrams of the most common shapes you encounter. We like that the angles are indicated for each diagram.
  • This study guide from Angelo University goes over the Lewis structures, the types of bonds you will encounter, and talks about exceptions and resonance structures. There are plenty of examples, and this material will help you explore additional concepts.
  • This molecular geometry guide from Oklahoma State University sums up all the different structures with a detailed table. This is a great resource if you need help with understanding how angles are determined.
  • This comprehensive guide explores the Lewis dot structures and shows you how to place the dots around the atom symbols.
  • You can test your molecular geometry knowledge with these flashcards and quiz questions.

Interactive 3D Tool

The University of Colorado has an online interactive tool that lets you create 3D models of different molecules. You can select different types of electron groups and see how they will connect to an atom.

This tool will show you the angle of the bonds and show models for different molecules if you don’t want to create one from scratch.

This visual tool would be very useful in a classroom when teaching molecular geometry chart information, but you can use it to try and recreate the thirteen different structures that exist in molecular geometry or to try recreating different specific molecules.

Molecular geometry is a fascinating subject. Studying molecular geometry can seem complex at first, but things become easier once you become familiar with the different structures that exist and understand which factors will influence how a molecule is shaped. Make good use of the study guides listed above, and take the time to play with the interactive 3D tool to try different things.


Everything You Need to Know About the Calvin Cycle

The Calvin Cycle occurs during photosynthesis and consists of light independent redox reactions that convert carbon dioxide into glucose. This conversion happens in the chloroplast, or more specifically the stroma of the chloroplast. The chloroplast region is an area between the thylakoid membrane and the inner membrane of the organelle which is typically located in the leaves of plants.

This cycle used to create carbon sugars, mostly, was discovered by Melvin Calvin, Andrew Benson, and James Bassham in 1950 at the University of California. The used radioactive material to trace the pathways carbon atoms took during the carbon fixation step in plant life.

You’ve probably heard the Calvin Cycle called a few other names including the CBB Cycle, C3 Cycle, and dark reactions to name a few.

This process of carbon fixing by plants is essential to all life on the planet. Most new organic growth stems from plants converting carbon to sugars either directly or indirectly. Other plants, or animals, can use these sugars to forms more complex sugars and amino acids when they consume them. It all stems from little plants working day and night to capture light and water.

A Technical Take on the Calvin Cycle

The Calvin Cycle

The Calvin Cycle occurs during photosynthesis and is repeated until it forms a glucose molecule. Photosynthesis goes through two stages to create food and building materials for plants to grow. During the first stage, chemical reactions from light produce ATP and NADPH. The second stage is when the Calvin Cycle takes place. In this stage, carbon dioxide and water get converted to organic materials like glucose. These reactions are called dark reactions which confuses people, but they do not take place at night.

The short explanation of the Calvin Cycle is that it begins with carbon fixation. Carbon dioxide molecules are plucked out of the air to produce glyceraldehyde 3-phosphate. RuBisCO, an enzyme found abundantly around the planet, brings on the carboxylation of a 5-carbon compound and provides a 6-carbon compound that halves itself form two 3-phosphoglycerate. The enzyme phosphoglycerate kinase uses the phosphorylation to create biphosphoglycerate.

Next, the enzyme glyceraldehyde 3-phosphate dehydrogenase uses the reduction of biphosphoglycerate by NADPH. This is called the reduction reactions. Eventually, when the cycle ends, the reactions and reductions produce one glyceraldehyde 3-phosphate molecule per every three carbon dioxide molecules.

That’s a lot of massive words. What that means is the plant uses light and water to convert carbon dioxide into nutrients and oxygen. It takes six turns on the Calvin Cycle for the plant to produce a single glucose molecule. Now that we simplified the process, let’s look at the chemical equation for the Calvin Cycle:

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)

The Simplified Function of the Calvin Cycle


How plants create sugar from sunlight, water, and carbon dioxide is complicated as you probably noted from the previous section. However, plants toil away day and night creating glucose, starch, and cellulose so they can grow. The Calvin Cycle plucks carbon molecules right out of the air and creates new plant growth.

The Calvin Cycle is vital to every ecosystem, and it reaches far beyond the plants using it. Plants are the building blocks of all the food in any ecosystem. Herbivores eat plants for energy and growth while carnivores eat herbivores for the same reasons. In the end, everything goes back into the ground and plants start the process all over again.

If plants stopped all their hard work tomorrow, it would only take a few days for animals to start feeling the effects and starving. Herbivores lose their food right away. Carnivores would follow behind the herbivores. Plants make most of the basic building blocks we all need to continue life as we know it. Without their hard work, we’d all be doomed.

While plants are supplying us with the building blocks, we need to continue living, and they help out the environment in other ways. Because the Calvin Cycle depends on carbon dioxide, plants indirectly play a role in regulating carbon dioxide and other gases proven to be harmful to the atmosphere. Plants perform an essential role in helping us clean the air we breathe.

The Calvin Cycle Step by Step

Calvin Cycle step by step

Carbon fixation is the first step. We explained it in brutal technical detail above, but let’s look at it in simpler terms in this section. A carbon dioxide molecule is plucked from the air and combined with a five-carbon acceptor molecule called ribulose-1,5-bisphosphate, or RuBP for short. The result is a six-carbon molecule.

The six-carbon molecule is split in half to form a set of new carbon molecules called 3-phosphoglyceric acid, or 3-PGA for short. The new three-carbon molecules are catalyzed by an enzyme called RuBisCo. This creates the simple sugar molecules the Calvin Cycle needs for stage two. On a side note, because it is used by every plant during photosynthesis, the RuBisCo enzyme if the most common catalyst on Earth. The result of this step is passed on to the next phase.

Step two of the Calvin Cycle is called the reduction step. The 3-PGA molecules created in the carbon fixation step are used in phase two to develop glyceraldehyde-3 phosphate or G3P for short. G3P is a simple sugar. This process uses energy and reactions captured during light-dependent stages of photosynthesis.

This step is called the reduction step because electrons are stolen from molecules created during photosynthesis and given to our new sugars. In chemistry, when you take electrons from a molecule, it’s called a reduction hence the name of this stage. Technically, the electrons are donated and not taken. Taking electrons by force is called oxidation, and that’s not what happens in this stage.

At this point, our plant has created sugar it can store for a long time and use for energy. Anything that eats this plant gets to take advantage of these sugars as well including humans. The plant may choose to use these stored molecules to form new plant materials or repair itself, but that’s not part of the Calvin Cycle so we won’t get into it. This is the end of the sugar-producing phase of the Calvin Cycle.

The final stage of the Calvin Cycle is called the regeneration step. Some of the G3P are held back and not used to make sugars. Instead, they are used to revitalize the five – carbon compound the Calvin Cycle needs to start the process over again. It takes six carbon molecules to make glucose, so plants have to go through the Calvin Cycle six times to make one glucose molecule.

Once the plant has completed this cycle six times, the Calvin Cycle ends and begins again. So, technically, the Calvin Cycle is all three steps done six times each. Plants repeat this process over and over during daylight hours. At night they continue to work making various compounds that don’t require light. This makes plants the most efficient lifeforms on the planet.

Bonus Information About Plants and Their Internal Food Factories


We usually consider waste products bad or at least not edible. However, we need the waste materials plants to produce to survive. An essential waste, or by-product, plants produce is oxygen. While plants are using water and carbon dioxide to make sugars, they release oxygen into the air around them as a waste product.

The delicious fruits and vegetables we all enjoy get most of their flavor from the carbon sugars plants store for energy. From the crunchy stalk of the celery plant to the succulent meat of the peach, plants developed all using just carbon dioxide, water, sunlight, and a few minerals leeched from the soil. I think we can assume these tasty treats are little gifts from the plant kingdom.

The tiny organelles called chloroplasts on the surface of a plant’s leaves can move. Ok, they can’t move individually, but in many plants, they can turn the leaf, so it gets better exposure to sunlight. These plant-based solar cells help capture sunlight so being able to point yourself in the sun makes sense. Some plants take it to another level and bend their stalk or branches to help reach the sunlight.

Some Final Notes

photosynthesis diagram

The fantastic plants we ignore all around us are vital to our survival. They use energy from the Sun in little energy reactors called chloroplasts to do all sorts of cool things. If you glance at the bigger picture and oversimplify it, plants take light from the Sun and turn it into carbon sugars they can store for long periods of time. We could call them solar powered batteries if we want to be humorous about the process.

Plants pitch in and help everywhere they can from cleaning the air to enriching the soil they grow in for the next plants. Plants give us so many things from apples to steak. Without plants toiling away at the bottom of the food chain, nothing in the top of the food chain could survive. Every food we consume comes from plants either directly or indirectly.

Your Guide To Your First Earthworm Dissection

Earthworms play essential roles in many ecosystems. They help introduce oxygen to the soil and mix it up. As they tunnel through the ground, they enrich the soil and push it toward the surface where it’s easier for plants to get to the nutrients. You can see the organs that help these worms do their jobs by dissecting an earthworm.

Safety First

woman showing earthworms

Safety is critical in all aspects of our lives. It may seem trivial in a controlled environment like a school biology lab, but it’s not, and all safety rules should be followed. They are in place to protect you and your classmates, so don’t skip any regulations just because you think it will be ok or those rules don’t seem to apply to your circumstances. The basic common-sense rules are:

  • Wear safety gear when necessary like goggles, gloves, and aprons.
  • Most preserved specimens contain formaldehyde, so wash them first.
  • Do not play with lab equipment or instruments such as scalpels and scissors.
  • Do not eat any parts of your specimen. Yes, there is an apparent reason for this rule.

Your lab should have the rules and safety measures available plus your instructor will go over them with you. Don’t assume the only rules are the ones we list here. The type of lab and type of specimen determine the rules. Ask for a copy of the rules if you don’t see one posted in the lab. Your teacher should be close by most of the time to help you guide you as well.

Always wear safety goggles and gloves. If you have to carry a sharp instrument, hold it with the pointed end pointing down and away from your body. Don’t rush or run while holding a scalpel or scissors. Never carry a knife or scissors by any part other than the handle. Scalpels are razor sharp, and it only takes a split second for them to cut you open.

Keep your station clean and tend to any spills immediately unless they pose a breathing hazard. Dispose of any blades, gloves, aprons, and specimens according to the established rules in your lab. Your teacher will probably explain all the rules to you, but don’t wait to ask if you aren’t sure what to do. Teachers are there to help educate you and keep you safe.

Earthworm Dissection Guide

earthworm dissection

Earthworms are great for helping you understand simple organisms and basic anatomy. They’ll help you get a grasp on lab safety before you progress to larger specimens like pigs or frogs. As a bonus, they’re small and soft, so handling them is much more comfortable as well.

The first step is to examine the exterior of the earthworm. Earthworms are segmented works, so they look like a long stack of small rings. They don’t have a head or any limbs, but they do have a fascinating exterior anatomy to study. The anterior end of the earthworm is a little fatter than the posterior. When you locate the anterior end of the work, pin it to the dissecting pan or tray.

Earthworms are annelids which means their bodies are composed of multiple ring-like sections or segments. This part may not be on your teacher’s list, but it’s always interesting to count the segments while you study the exterior anatomy of the earthworm. While you count, notice the small setae on the ventral surface. These little bristles help the worms move through the dirt with ease.

Each segment along the worm’s exterior has small pores. These pores excrete the sticky film you find when you run your finger along a live worm. You may need a magnifying glass or small microscope to see them. It depends on the size of your earthworm specimen and your eyesight as well.

From the anterior end of the worm, count your way down to segment fourteen. Typically, this is where the oviducts are located. The oviducts release the eggs when the worm reproduces. The exciting part is the next segment after the oviducts; it contains the sperm ducts. Earthworms have both male and female reproductive organs.

Further down the worm at segment 31 is the clitellum. It secretes a sticky mucus that binds two earthworms together while the mate. It develops a cocoon to hold the eggs and sperm after mating is finished. Earthworms are simple worms, but fantastic at the same time. Their exterior anatomy is fascinating to study.

Earthworms are hermaphroditic which means they have both female and male reproductive organs. Eggs come from the ovaries inside segment fourteen, sometimes thirteen. It can be hard to count the segments on small worms. Worms have testes which can form in segments near the oviducts. Study these segments and see if you can find the reproductive organs on your specimen.

When worms mate, they get stuck together briefly to help keep the reproductive organs aligned. Sperm from both worms travels into the other worms seminal receptacle. The clitellum creates the cocoon which moves along the outside of the worm to collect the semen and the eggs. The eggs are fertilized outside the worm in the cocoon.

By now, you should have a good understanding of the exterior anatomy of your earthworm specimen. Remove the pin from the anterior end of the earthworm and place it on its ventral side, then put the pin back in the anterior end of the worm. The ventral side of the worm is a little flatter than the dorsal side, and it may be a lighter color.

Carefully and slowly make a shallow incision using your scalpel from the anterior end of the work to the clitellum. Never cut toward your body or fingers. Be extra careful and keep the incision shallow, so you don’t cut into the worm’s digestive system and internal organs. Use your forceps to spread the worm open and pin the sides of its body to your dissection pan or tray.

The inside of the worm should be exposed now. You may want to lightly sprinkle water over the worm to keep it from drying out while you study the inside of it. The interior part of the walls is called the septa. See if you can tell the difference. If possible, ask your teacher to point them out and help you see the different layers.

Now, the internal digestive organs should be exposed and available for study. Starting with the mount on the anterior end of the worm, locate the organs. The first organ you see is the pharynx. The worm’s esophagus protrudes from the pharynx. About halfway down your incision are the crop and gizzard. Skip the other organs for now and find those two.

The crop is essentially a stomach. It stores food until the food is moved to the gizzard which grinds it up. The food leaves the gizzard and goes into the intestine, much like it does in humans, and travels to the anus. Along the way, the worm’s intestines absorb nutrients from the food the gizzard crushed and ground up. Earthworms don’t eat dirt. The consume organic materials found in the soil.

Make your way back up to the crop. If you look above the crop on the anterior side, you’ll find five pairs of aortic arches. This is the worm’s version of a heart. The hearts are located around the esophagus, and they connect to the dorsal blood vessel. That’s the worm’s version of an artery. Most earthworms can take direct damage to half their aortic arches and live.

Move your attention back to the pharynx at the anterior end of the worm. Locate the cerebral ganglia beneath the pharynx on the dorsal side. You may need to use your forceps to move some organs around to get a good look at it. The ventral nerve starts at the cerebral ganglia and runs the length of the worm. It may be hard to see if it is too small.

They are simple creatures speaking purely on their anatomy, but how their bodies and mating works are truly amazing. If you have time, go back over this tutorial again and study the worm longer. When you finish exploring, make sure you clean your workstation and dispose of your specimen correctly. Dispose of your lab gear according to the lab rules. Wash your hand thoroughly with soap and water.

Some Final Notes


Earthworms are vital to the health of our soil. The improve drainage, help stabilize the land, and add nutrients to the ground. Worms feed on organic materials they find in the dirt. Their bodies use the nutrients they need and deposit what’s left back into the soil as waste. Fortunately for plants, that waste is usually nitrogen-rich along with other nutrients plants need to grow.

Their worm tunnels help loosen the soil which aids plants in root development. We could go on and on about the benefits of earthworms. If you follow our guide to dissecting earthworms and read our interesting facts along the way, we’re sure you’ll be able to dissect an earthworm specimen safely. You may even appreciate these simple creatures a little more when you are done.

14 + Practice Problems To Add To Your Genetics Study Guide

The study of genetics is fascinating, and it’s more than just the study of “where we come from.” An AP Biology test may cover integral information like Mendel’s Dihybrid Cross Experiment or general but essential genetics terms like asexual reproduction.

These genetics practice problems can be added to any teacher-written study guide or a great resource for any student who wants to make sure they have all the information they need while studying for a genetics test.

Since genetics is such a broad subject, it can be difficult to decide which genetics practice problems to add to your guide. It’s best to add a little bit of everything to ensure a thorough understanding of how genetics works in regard to Biology.

While some of our genetics practice problems might not be useful or relevant as others, you may pick and choose these questions to help “fill” your study guide and boost your overall knowledge of the subject.

A Few Tips For Studying

When studying for your genetics test, you are likely to encounter many practice problems that you need to figure out and show your work, such as the phenotype ratio. Creating flashcards for genetic vocab is another great way to memorize those terms easier.

While everyone has a learning style that works best for them, choose a study tool that will not only help you memorize the material but will also help you to understand it. The memorization of material has little use if you don’t know what you’re memorizing.

Another great way to add more information to your study guide is to form a study group and put everyone in charge of coming up with a few questions. Not only will this help everyone in the group retain more information, but it can break up the monotony that sometimes results from studying.

If you’re an instructor and putting together a study guide for students, why not allow each student to come up with a question (that they can answer) and add it to the study guide? It allows them to do a little research and interact with their peers.

14 Vocab Terms To Add To Your Study Guide


Sometimes the easiest and best way to learn genetics is to start with the basic genetic terms, and you might want to consider adding these terms to your study guide (or make some flash cards as we already recommended). There may be many more you want to add to your study guide, but here’s a start:

  • Genotype:The genetic makeup of a living organism
  • Phenotype:An observable trait or physical appearance (i.e., eyes)
  • Allele:A form of a gene
  • Gene:The basic unit of DNA
  • Homozygous:Alleles that are identical
  • Heterozygous:When alleles are different
  • Dominant Trait:Always present in the phenotype when present in a genotype
  • Recessive Trait:Only present in the phenotype when no dominant traits are in genotype
  • Punnett Square: A chart which shows all possible genotypes of a living organism from reproducing (or crossing over)
  • Incomplete Dominance:When two homozygous phenotypes combine and result into a heterozygous phenotype
  • Codominance: Two dominant traits that have equal representation in the results
  • Autosomal:Any chromosome not on the sex cells
  • Karyotype:A picture of all the chromosomes in a cell and arranged into pairs
  • Epistasis:One gene locus alters the expression of the second locus. Ratios are different from what’s expected.

When one gene locus alters the expression of a second locus. Ratios are often altered from the expected. One treatment act as a recessive because it is “hidden” by the second trait.

What Do You Know About Mendel?

Since Gregor Mendel’s research plays such an integral role in the genetics we know today, it’s important to understand his work. Take a look at these questions (with the answers) to see how much you know about Mendel and his work in the field of genetics.


Mendel used purebred plants in his experiments. What are two possible genotypes of a purebred plant?

A purebred plant only produces the same type of offspring when self-fertilized. The plants must be homozygous for two genotypes to be possible. One example is purple flowers: WW and white flowers: ww.

In his pea plant experiments, Mendel examined many traits, which included the height of the plant and flower color. Which of the following answers best represents the plants of the P generation?

  • 1Homozygous purple, homozygous tall x heterozygous white, homozygous short
  • 2Heterozygous purple, homozygous tall x homozygous white, homozygous short
  • 3Homozygous purple, homozygous tall x homozygous white, homozygous short
  • 4Homozygous purple, homozygous tall x heterozygous purple, homozygous short

If you selected “C” for your answer, you’re right.

What did Mendel call the traits that were not expressed in the F1 generation?

  • 1Recessive
  • 2Heterozygous
  • 3Incompletely dominant
  • 4Hybrids
  • 5null alleles

If you chose “A,” you’re correct.

Mendel is famous for his dihybrid cross experiment. How does the dihybrid cross differ from the monohybrid cross?

  • 1Monohybrid cross includes a single parent and dihybrid has two parents
  • 2Monohybrid cross produces one offspring, and dihybrid cross produces two
  • 3A dihybrid cross involves heterozygous organisms for two characters, and monohybrid is only one
  • 4Monohybrid cross is performed for only one generation, and dihybrid cross is performed for two
  • 5Monohybrid results in 9:3:3:1 ratio and dihybrid cross is a 3:1 ratio

The correct answer is “C.”

When Mendel performed his famous genetic experiment between pea plants, the pea cross (the offspring of the F1 generation) always looked like one of the two parental varieties. Why?

  • 1One phenotype was dominant over the other
  • 2Each allele affected the phenotypic expression
  • 3Traits blended together during the process of fertilization
  • 4No genes interacted to produce the parental phenotype
  • 5Different genes interacted to produce the parental phenotype

If you chose answer “A,” you are correct.

Mendel had many findings when he conducted his experiments with the pea plants. What was his most ground-breaking and significant conclusion?

  • 1There substantial genetic variation in pea plants
  • 2Traits are inherited in “discrete units” rather than the result of “blending”
  • 3Recessive genes are more common than dominant ones
  • 4Genes are composed of DNA
  • 5Organisms that are homozygous for recessive traits have numerous disadvantages

The correct answer to this practice problem is “B.”

More Questions On Genetics

Now that you’ve tested your knowledge on Mendel let’s take a look at some other questions that might be good to add to a study guide when preparing for a genetics exam.

If an individual has a genotype AaBbCCDdEE, how many unique gametes can be produced through independent assortment?

  • 14
  • 28
  • 316
  • 432
  • 564

If you’ve done your math right, the correct answer should be “B.”

Labradors are yellow, brown, or black. If a black female mates with a brown male, the results are as follows: all black puppies, half black to half brown puppies, or three-quarters black to one-quarter yellow puppies. The results of the colors of puppies indicate what?

  • 1Brown is dominant to black
  • 2Black is dominant to brown and yellow
  • 3Yellow is dominant to black
  • 4Incomplete dominance
  • 5Epistasis isiInvolved

The correct answer is “E.”

Continuing with the same question about Labradors, how many genes must be responsible for these coat colors in the puppies? 

  • 1One
  • 2Two
  • 3Three
  • 4Four

The correct answer for this question is “B.”

One more question involving the Labs. One type cross of black and black the results were: 9/16 black, 4/16 yellow, 3/16 brown. The genotype aabb must result in the following?

  • 1Black
  • 2Brown
  • 3Yellow
  • 4A fatal result

If you chose “C,” you are correct.

If the inheritance of the first genetic trait is not dependent on the inheritance of the second trait, what is this in reference to?

Your answer should be The Law of Independent Assortment.

What are the genotype and phenotype ratios of the following cross: Dd x Dd?

The genotype should be 1DD: 1Dd : 1dd

The phenotype should be 3 dominant: 1 recessive

In petunias, heterozygotes for one of the genes have red flowers. Homozygotes have purple or white flowers. When petunia plants with purple flowers cross with one that has white flowers, what percentage of the offspring will have red flowers?

  • 10%
  • 225%
  • 350%
  • 475%
  • 5100%

If you came up with 100% (E.) as your answer, you are correct.

If a woman has seven fingers on each hand and her husband and son have the normal amount of digits on their hands, what fraction of the couple’s other children would be expected to have extra digits? Treat additional digits as a dominant trait.

If your answer is 50%, you’re right.

Since genetics is such an in-depth study, we may not have covered all the topics that may be on your exam. Our practice problems, plus vocab words, should give you a good start and a great opportunity to practice what you already know about genetics.

Codominance: Definition, Examples, and Practice Problems

As you start learning more about genetics in AP Biology, you will learn about dominance and how it refers to the relationship between two alleles, which are variations of a gene. When there’s a dominant relationship between alleles, one of the alleles will “mask” the other to help and influence a specific trait.

You can explore this further by taking a look at complete dominance, which is when the phenotype of the heterozygote is identical to the dominant homozygote. Remember, the phenotype is an observable characteristic such as the texture of hair on a human, the length of fur on an animal, or the color of petals on a flower.

As your instructor talks more about complete dominance and the role it plays in the genetics of all living organisms, they will also discuss incomplete dominance. While there are some similarities between incomplete dominance and codominance, it’s important to remember that they are completely different and both play an integral role in genetics.

In this article, we will give you an in-depth explanation of codominance, the difference between incomplete dominance and a codominant relationship, give you a few examples, and a practice problem to try out, so you have a better understanding of this unique relationship.

A Brief Look At Mendel’s Law of Dominance and a Few Important Terms To Remember

Whether you’re just starting to learn about genetics in your Biology course or you need a little refresher (or help) to understand some of the basic concepts surrounding a dominant relationship going over Mendel’s Law of Dominance can be helpful. We will also define some important genetic terms to help us explain codominance a little better.

Since codominant and incomplete dominant relationships are similar and often mistaken for one another, it’s best to spend a little time going over Mendel’s Law of Dominance first (as a starting point).

Even if you’re just starting out your study of genetics, you’ve probably heard a lot about Gregor Mendel. His research was groundbreaking and everything we know about genetics today started with him.

Mendel is known for many of his experiments and findings, but he’s best known for his three laws, which include the law of segregation, the law of independent assortment, and the law of dominance (which we will discuss very briefly).

In his law, Mendel found that the dominant trait is always present in the offspring. When someone inherits two different alleles from each of the parents and the phenotype of only one allele is observable (such as hair or eye color), the allele is dominant.

When one parent has two copies of an allele (let’s call it “D”), which makes it dominant, and the other parent has two copies of allele “d” (which is recessive), the offspring inherits a “Dd” genotype and the dominant phenotype.

As you can see, we’ve tossed in a lot of vocab terms for genetics that can be a little hard to remember. While you might know what most of them are, it’s important to have a clear understanding (since they play such an integral role in dominant relationships).

Here are a few terms to know:

  • Allele:A different form of a gene (the DNA for a trait), variant
  • Heterozygote:Someone that has two different forms of a specific gene, one from each parent
  • Homozygous:Someone that has two identical forms of a gene, “true breeding” characteristic
  • Phenotype:Noticeable characteristics of the genetic makeup (such as hair, eyes, skin color)
  • Genotype:The genetic makeup of an organism, like the traits.

Now that you have the general concept of what a dominant relationship is and how it works, let’s see the difference between a codominant and incomplete dominant relationship.

What’s The Difference Between Codominance and Incomplete Dominance?

Even though Mendel played an integral part in observing dominant relationships, codominant and incomplete dominant relationships are considered to be non-Mendelian inheritance patterns.

What Is Codominance?

In a codominant relationship, neither allele is recessive or masked by the other allele (which make the pair that code a characteristic). Blending plays a role in a codominant relationship, and both alleles are equally expressed, and their features are both present (and seen) in the phenotype.

In a way, you could think of codominance like “co-parenting,” where each parent plays an equal role. In a codominant relationship, both alleles are passed down from one generation to the next, rather than being bred out.

How Does Incomplete Dominance Differ?

We know what complete dominance is and incomplete (or partial) dominance may be a lot like it sounds. Incomplete dominance refers to when one allele for a certain trait is not entirely dominant over its counterpart (the other allele). The offspring end up with a combined phenotype.

The traits of each parent are neither dominant or recessive and a third phenotype results. The alleles don’t actually blend, but the traits appear to be mixed, so many people refer to the result of incomplete dominance as “blended.”

As you can see codominant and incomplete dominant relationships are very similar. While one has actual blending going on in the offspring, the other appears to be; you can see how some people might assume they are the same, right?

A simple way to explain the differences between the two is that in incomplete dominance, the traits of the offspring are unique and similar to the dominant traits (but still a trait of its own). Such as black feathers and white feathers produce silver feathered offspring.

A codominant relationship will produce offspring that has both traits visible. You can get a better idea of how this works in the examples below.

Examples Of Codominance

The easiest and best way to get a better understanding of a codominance is to take a look at real-life examples and here are a few:

Codominance In Flower Colors

If you know anything about incomplete dominance, you might be familiar with red and white flowers having offspring with pink flowers.

two tone roses

Let’s see how it differs in a codominant relationship. If two plants were crossed to produce a yellow and blue flower (and the alleles for petal color were dominant), the offspring would be yellow with blue spots or blue with yellow spots. Do you see how each allele plays a significant role in the color?

Codominance In Animals

There are many examples of incomplete dominance in animals. A spotted dog mates with a solid colored dog. The offspring would have some spots (kind of “in-between”) from both parents. The same idea goes for fur length and the color of feathers.

papillon dog and chihuahua

A popular example of a codominant occurrence is when a white homozygous horse mates with a homozygous red horse. The offspring ends up with a roan coat, which is a mixture of red and white hair (each strand of hair is either white or red). There are other animal examples, that are similar, that include cats, cattle, and dogs.

Codominance In Humans

When people think of incomplete dominance in humans, they often use wavy hair as an example, which is a result of a parent with straight hair and another with curly hair. Skin color, height, size of hands, and pitch of voice are all examples of incomplete dominance in humans.

So, what’s a good example of a codominant inheritance in humans? The most common example is in regards to the AB blood type. Human blood type follows the ABO system, which refers to the three different blood groups: A, B, and O.

The alleles encoding the A and B groups are dominant, and the O group is recessive. The results may be as follows:

  • AA (Blood Group A)
  • AB (Blood Group B)
  • AO (Blood Group A)
  • AB (Blood Group AB)
  • BB (Blood Group B)
  • BO (Blood Group B)
  • AO (Blood Group A)
  • BO (Blood Group B)
  • OO (Blood Group O)

In the AB blood type, for example, the “A” type blood cells have one kind of antigen, and the “B” type have another. While antigens typically alert the body of a “foreign” blood type attacking the immune system, people with AB blood have both antigens and their immune system cannot be attacked by either type; this is why AB blood is considered to be “universal.”

Ready To Test Your Knowledge?

Are you ready to see how much you know about codominant inheritance? Check out this practice problem and select the right answer.

Which of the following is NOT an example of a codominant relationship?

  • 1Offspring with AB blood type, whose parents have blood types A and B
  • 2A calf has red and white hairs, and one parent is white while the other is red
  • 3A child with brown eyes has a parent with blue eyes, and the other has brown eyes
  • 4A flower has red and white petals (it’s the offspring of red and white flowers)

If you chose “C,” you’re correct.

We’ve talked a lot about animals with roan coats. Here’s your question:

Is it possible for red offspring to be born to a white horse that mates with a roan horse?

If you said, “No,” then you’re getting a good understanding of codominant inheritance.

Incomplete Dominance: Definition, Examples, and Practice Problems

You may already know that in the study of genetics, dominance refers to the relationship between alleles, which are two forms of a gene. In a dominant relationship between alleles, one allele “masks” the other and influences a specific trait.

When the phenotype (the observable characteristic) of the heterozygote is identical to the dominant homozygote, the relationship is considered to be “complete dominance.” Since genetics is full of variations and changes, complete dominance isn’t always the outcome but rather incomplete dominance.

In this article, we’ll give you an in-depth explanation of incomplete dominance (also known as partial dominance), some examples, and a practice problem so that you can try out on your own, so you can gain a better understanding of this type of relationship.

A Quick Look At Important Terms

concentrated doctor working with virtual screen

As you study genetics, you may find that it’s difficult to remember all the of the terms and what they mean. Before you can completely understand incomplete dominance, it’s a good idea to go over some basic genetic terminology.

  • Gene: The DNA for a trait
  • Allele: A different or variant form of a gene
  • Heterozygote: An individual with two different forms of a specific gene, one from each parent
  • Homozygote: An individual with two identical forms of a gene, results in true breeding for a characteristic
  • Phenotype: Observable characteristics of the genetic makeup
  • Genotype: The genetic makeup of an organism, such as traits

Now that we’ve reviewed a few of the genetic terms that you’re likely to see frequently when learning about partial dominance let’s move on to the concept of partial dominance.

Mendel’s Law of Dominance

Gregor Mendel is often referred to as the “Father of Genetics” because without his experiments, persistence, and years of research we probably wouldn’t have a good understanding about who we are or why we share traits with our ancestors. Mendel created three “laws” that he is known for: the law of dominance, the law of segregation, and the law of independent assortment.

To get a better understanding of partial dominance, we’ll take a closer look at Mendel’s “Law of Dominance.” In this “law” Mendel found (through his years of experiments) that the dominant trait is the trait whose appearance is always in the offspring. As we mentioned earlier, dominance is the relationship between the two alleles.

If someone inherits two different alleles from each of the parents and the phenotype (such as hair or eye color) of only one allele is noticeable in the offspring, then that allele is dominant.

If one parent has two copies of allele “A” (which would be dominant) and the other parent has two copies of allele “a” (which would be recessive), then the child will inherit an “Aa” genotype and still display the dominant phenotype.

Now that we have a full understanding of the dominance relationship between alleles, let’s see how the partial dominance differs.

Incomplete Dominance: What Is It?

We understand complete dominance, but you might still be wondering how partial dominance differs. Is it much like the name suggests? Partial dominance is when one allele for a specific trait is not entirely dominant over its counterpart (or the other allele). The result, which is seen in offspring, is a combined phenotype.

What does this mean? The traits of each parent are neither dominant or recessive. In a partial dominance relationship, between two alleles, a third phenotype is a result and is a combination of phenotypes of the two homozygotes; this is often referred to as an “intermediate form of inheritance.” The alleles do not blend, but partial dominance is often referred to as “blending” because traits are mixed and appear to be “blended.”

Examples of Incomplete Dominance

A better way to understand partial dominance is through examples and here are a few:

Snapdragon Flowers

A common example of partial dominance that many instructors of Biology use in the genetics unit are a snapdragon flower. In this example, the Snapdragon is red or white.

If a red homozygous snapdragon is paired with a white snapdragon (which is also homozygous), the hybrid result would be a pink snapdragon. Here’s how it the partial dominance looks when broken down:

The genotypes are Red (RR) x White (rr) = Pink (Rr)

When the first offspring (F1) generation, which is all pink flowers, cross-pollinates, the resulting flowers in the F2 generation consist of all the phenotypes: ¼ Red (RR): ½ Pink (Rr): ¼ White (rr). The phenotypic ratio is 1:2:1.

If the F1 generation cross-pollinates with the “true breeding” red flowers (homozygotes), the F2 generation will result in red and pink flowers (half-red and half-pink); the phenotypic ratio is 1:1.

If the F1 generation cross-pollinates with “true breeding” white flowers, the F2 generation will result in white and pink flowers (half of each and a phenotypic ratio of 1:1).

In the case of partial dominance, the intermediate (or 3rd ) trait is the heterozygous genotype. The pink snapdragon flowers are heterozygous with an Rr genotype, and the red and white flowers are homozygous for flower color with genotypes RR and rr (or red and white).

While snapdragon flowers are a common example, you can find the same results with red and white tulips, roses, and carnations.

Incomplete Dominance in Animals

Just like plants and humans (which we’ll give an example of briefly), partial dominance can occur in animals; as it can occur in every living organism.

Let’s look at an example of rabbits. If a breed with long fur, like an Angora rabbit, mates with a breed with short fur, like a Rex rabbit, the offspring is likely to have fur that is in the middle; not too long or too short.

two rabbits

Andalusian chickens are also a popular example of partial dominance in animals due to their unique blue-ish feathers. The chickens don’t always have slate blue feathers, but it is often a result of a white rooster mating with a black hen. Since both parents have the inheritance of blue alleles (about 50%), the offspring is likely to have feathers with a splash of blue.

If you consider cats and dogs, there are usually some cats or dogs that have more markings than one of the same breed. When a heavily spotted or market dog or cat marks with a mate that has solid-colored fur (and no markings), the offspring is likely to have some markings but not the same as either parent.

Partial dominance can apply to the length of tails, the color of fur, and many other phenotypes in animals.

Incomplete Dominance in Humans

By now, you’re probably able to see a pattern in how partial dominance works in genetics. It’s a complex idea, but when you break it down it’s not as complex as some people make it, right?

Consider some ways that partial dominance may occur in humans. Like the fur length on an animal, the child of one parent with curly hair and the other with straight-hair is likely to have wavy hair. Both straight and curly hair is dominant, but neither one dominates the other.

Diseases like sickle cell disease or Tay-Sachs disease is another example of partial dominance in humans. Skin color, height, voice pitch, and even the size of one’s hands can all be attributed to partial dominance.

Think about your own features. Are you a carbon copy of one of your parents or do some of your features sit “in the middle” and are a result of partial dominance?

A Practice Problem For Incomplete Dominance

Whether you want to study up on partial dominance or just want to play around with some scenarios and see what you come up with, take a look at a few of these practice problems.

A cross between a bird with blue feathers and a bird with white feathers produces offspring with silver feathers. The color of the birds is determined by only two alleles.

  • 1What are the genotypes of the parent birds?
  • 2What is the genotype of the bird with silver feathers?
  • 3Can you figure out the phenotypic ratios of the offspring of two birds with silver feathers?

The answers are as follows. How did you do?

The answer for #1 is BB (homozygous blue) for the bird with blue feathers and WW (homozygous white) for the bird with white feathers.

The answer to #2 is one blue allele and one white allele. Since neither allele is dominating another, we get a “blend” which results in the bird with silver feathers.

To figure out #3, you need to fill out a Punnett Square. Silver x silver = BW x BW. Your results should be 25% of offspring are homozygous white (WW), 25% are homozygous blue (BB), and 50% are hybrid, which means they have silver feathers.