Author: Biology Junction Team

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.