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.