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.

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