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.

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.

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.

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

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.

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.

Biological magnification is a rising concern amongst researchers who examine the ways that chemicals and pollutants may have long-term effects on ecosystems.  In this article, we’ll dive deep into what it is and the impacts it’s already had on our environment. 

Biology researchers and students are likely familiar with the field of ecotoxicology, or the study of how chemicals and toxins affect ecosystems and their organisms.  In this field, the term biological magnification is frequently used to describe the amplified concentrations of these substances as you move up through the food chain.

Also fittingly called bioamplification or biomagnification, this process explains why harmful substances like have metals, or chemicals found in fertilizers or pesticides, present in even the largest, carnivorous predators.

In this article, we will discuss the process of biomagnification and how it works.  We will define the terminology, and then give real-life examples and case studies documenting how chemicals travel through soil, water, and smaller organisms to eventually make their way to the top of the food chain in large concentrations.  

What is Biological Magnification?

Put simply; the term biological magnification is used to describe the process by which substances used in farming or produced in industrial waste make their way into and up the food chain.

We see increased levels of these toxins and chemicals accumulating through the trophic levels of the food chain thanks to this phenomena.

Pesticides, fertilizers, and heavy metals from industrial waste are some of the most common culprits who contribute to the problem.

Typically, the materials are carried through water sources like rivers, lakes, and streams as a result of surface runoff where they are then ingested by aquatic animals like frogs or fish.  These small organisms are then preyed upon by predators higher up in the food chain, like birds, larger fish, or animals, which is how these same substances make their way into their body.

Many of these toxins and chemicals are fat soluble and get stored in their internal organs or fat tissue.  This results in an accumulation of the substance over time and in greater concentrations the higher up the food chain you go.  This phenomenon is called food chain energetics.

Although biomagnification doesn’t always have a direct effect on living organisms, long-term exposure to harmful chemicals may result in unpleasant and irreversible side effects that could threaten a species.

Biological Magnification vs. Bioaccumulation

It’s important to note that there is a significant difference between biomagnification and bioaccumulation.  Although some may use the words interchangeably, they actually describe different scenarios in an organism.

Biological magnification specifically refers to increasing concentration of materials in each higher link in the food chain.  However, bioaccumulation examines the increased presence of a particular substance inside a single organism.

While the two processes may be interconnected, for the purpose of this article it’s important to differentiate the terminology to understand the real-life examples and practice.

Examples of Biological Magnification

There are numerous, well-documented examples of biomagnification where researchers find high concentrations of chemicals in apex predators.  Many of these studies also demonstrate the potential negative consequences of this build up over time. Here are a few examples.

Bald Eagles

During World War II troops faced a plethora of health issues, including outbreaks of malaria, body lice, typhus and bubonic plague spreading through mosquito bites at encampments throughout the world.

DDT is a pesticide that was developed to kill these biting bugs to help control the spread of these diseases, and following the war had agricultural applications.  Farmers used the product on their crops to control pests, and it was both popular and widespread thanks to its low cost and easy application.

It was approved as being safe and effective by the EPA at the time because there did not appear to be any harmful side effects of ingesting the chemical in animals or humans.  However, this did not take into account the possibility of biomagnification. 

DDT doesn’t break down over short periods of time in the environment and is a substance that gets stored in the fatty tissues of animals who consume it.  This became particularly problematic for bald eagles.

A predator near the top of the food chain, bald eagles were consuming large quantities of fish who had been affected by the chemical.  Runoffs from farms hit the waterways, and DDT infiltrated aquatic plants and animal life, and the eagles ingested the chemical with each meal they ate.

Over time, the chemical disrupted their ability to lay eggs with strong shells, causing the bald eagle population to decrease to the point of near extinction.  In 1940, Congress stepped in to pass an act to protect the species, but DDT wasn’t banned until 1972.

It wasn’t just species of eagles affected.  Other predator birds like brown pelicans and peregrine falcons saw the same side effects.  The thinning off the eggshells made incubation and hatching near impossible and also threatened these bird populations.

Fish and Pregnancy

Another notable example of biomagnification is in predator fish.  Species like Shark, Swordfish, Orange Roughy, Tuna, King Mackerel, or Tilefish contain proportionally larger levels of toxic mercury than smaller fish and shellfish.

In fact, the levels are so high that the FDA advises that pregnant women avoid consuming these species for fear of exposing developing fetuses to levels that may cause nerve damage.

How does this toxicity occur?  Mercury is introduced into the ecosystem in one of two ways.  As a naturally occurring element, it can leach from rocks and volcanoes into our water supply over time, but those natural changes are not likely to significantly impact the environment.

However, when you take the natural occurrences and combine them with human contributions through coal-burning power plants which impact the air, rain, soil, and water around these facilities, the mercury levels rise drastically.

As we now know, once an element enters the water supply, it’s inevitable that it gets ingested by aquatic life at every level of the food chain.  When plankton and small crustaceans that make up the majority of the diet of the larger, predatory fish have moderate levels, then the species who eat them will have a compounded effect.

For example, according to the FDA, the average amount of mercury found in a serving of scallops is 0.003 parts per million.  Lobsters, one of the main predators of the scallop have a concentration of 0.107 parts per million.  

Monkfish love dining on lobster, and have an average of 0.161 parts per million of mercury in their system, and shark and swordfish at 0.979 and 0.995 parts per million respectively regularly dine on monkfish.

In this example, it’s easy to see how quickly the effects compound and how concentrated they become with only four steps up the food chain ladder.

What Causes Biological Magnification?

Although biomagnification is a natural phenomenon that happens in all organisms, the instances where it is worrisome are largely due to anthropogenic factors.  Materials that humans introduce into the environment can cause unexpected and hazardous side effects and typically fall into one of the following subcategories.

Organic Contaminants

We live in an age where the word organic is closely correlated with natural and healthy, but too much of anything could be bad.  Organic elements like phosphorus, nitrogen, and carbon are necessary for survival, but if they appear in excessive quantities in ecosystems, they may cause eutrophication.

Eutrophication is a phenomenon when an organism that thrives in these conditions, like algae, for example, experience exponential growth and suddenly have an overwhelming population.  This can then disrupt the ecosystem and kill off other organisms because there aren’t enough resources, like oxygen, to go around.

Waste

Waste produced from manufacturing plants, factories, and other industrial enterprises can release waste and toxins into the air and water that contribute to the problem. 

Agricultural and Industrial

Chemicals introduced into the environment from inorganic pesticides, fungicides, fertilizers and herbicides that mix with our natural water sources due to runoff when it rains release toxic elements as well.

Plastic Pollution

Not only does plastic physically impact our environment, often ending up in our oceans and disrupting marine ecosystems, but it can also leach toxic chemicals into water too.

For example, Bisphenol A, or BPA, has made headlines recently as a substance that can produce a range of health conditions in humans that is used in making plastic water bottles.  It is one of the leading chemical pollutants in the environment.

Heavy Meals

As we discussed in our earlier case study, heavy metals that enter our water sources can wreak havoc on the ecosystem.  Mining activities are sometimes at fault for releasing deposits that can pollute aquatic plants and contaminate water sources with elements like zinc or cobalt.  

Potential Negative Effects of Biological Magnification

DDT and mercury aren’t the only hazardous substances that have the potential to biomagnify.  Substances like polychlorinated biphenyls (PCB’s) that can impair reproductive systems, heavy metals, polynuclear aromatic hydrocarbons which are a known carcinogenic, cyanide, and selenium have been extensively studied and proven to have similar outcomes.

There are dozens of potential adverse effects to our environment, including but not limited to:

• Reproductive implications for marine and other animal life
• Killing coral reef ecosystems
• Disrupting the natural food chain as species die off

There is also a significant risk of health impacts on humans who consume many of the organisms affected by this process.  They include an increased risk of the following:

• Cancer
• Kidney failure
• Liver disease
• Birth defects
• Brain damage
• Respiratory disorders
• Heart disease

Final Thoughts

Bioamplification isn’t a new phenomenon, but the humans have introduced pollutants to the environment that makes it a threat to the ecosystem and our food sources.  Understanding how and why it occurs is the first step to combating the problem and preventing the destruction over time.

Conversations and advocacy for sustainability need to continue to ensure the long-term health of our environment.