Incomplete Dominance vs. Codominance: Videos & Practice Problems

Incomplete dominance and codominance are two patterns of inheritance. Incomplete dominance occurs when heterozygous individuals exhibit a blended phenotype, such as pink flowers from red and white parents. In contrast, codominance involves both alleles being equally expressed, as seen in flowers with patches of red and white. A classic example of codominance in humans is blood type, where alleles IA and IB are co-dominant, resulting in AB blood type with both A and B surface molecules on red blood cells. Understanding these concepts is crucial for grasping genetic inheritance.

Incomplete Dominance

In this video, we're going to begin our lesson on incomplete dominance versus codominance. We're going to start off by introducing incomplete dominance in this video, and then later in a separate video, we'll introduce codominance. Incomplete dominance is a pattern of inheritance where heterozygous individuals show a blended phenotype that is really just an intermediate of the phenotypes from the 2 alleles. We will look at incomplete dominance in a red and white flower cross, and how this incomplete dominance results in pink baby flowers.

On the left-hand side, we have a table of the specific allele and the phenotype that they lead to. There are only 2 alleles in this example. There is the r₁ allele, which leads to a red flower phenotype, and then there is an r₂ allele, which leads to a white flower phenotype. Notice that there is not a third allele here. With incomplete fertilization of a red flower with a white flower, note that the red flower is homozygous for the r₁ allele and the white flower is homozygous for the r₂ allele. When we perform this cross, we end up getting heterozygous individuals. We get r₁ and r₂ in each of these boxes, resulting in a heterozygous allele.

Remember, the r₁ allele says, "make red flower", and the r₂ allele says, "make white flower". However, when we have a heterozygous individual, it shows a blended phenotype that is an intermediate of the phenotypes from the 2 alleles. Instead of getting a red flower or a white flower with the heterozygous individual, we get a pink flower. Notice that neither the r₁ nor the r₂ allele says, "make pink flower". They either say "make red flower" or "make white flower". But with incomplete dominance, we get neither red flowers nor white flowers. Instead, we get an intermediate blend of pink flowers. That is the pattern of inheritance referred to as incomplete dominance, characterized by a blended phenotype that is an intermediate of phenotypes.

This year concludes our introduction to incomplete dominance, and we'll be able to compare this to codominance in our next video. I'll see you all there.

Codominance

In this video, we're going to introduce codominance. Codominance is a pattern of inheritance where heterozygotes with two different alleles equally express both phenotypes from each allele in patches. In other words, codominance is when two different alleles dominate together. When you take a look at the word codominance, the root 'co' in front of dominance is a root that means together, just like 'cooperate' has the root 'co' which means together. Codominance means that they're going to dominate together, where one allele does not mask the expression of the other allele. Instead, they dominate together. We're going to take a look at an example of codominance in red and white flowers, which is very similar to our last example in our last lesson video, except instead of showing an incomplete dominance pattern of inheritance, here we're showing a codominance pattern of inheritance.

Once again, we're taking a look at these two different alleles, the r₁ allele, which is associated with a red flower phenotype, and the r₂ allele, which is associated with a white flower phenotype. When we cross a homozygous red flower with two r₁ alleles and a homozygous white flower with two r₂ alleles, we're going to get this Punnett square that we see here where we get r₁ and r₂ heterozygotes as the offspring, just like what we saw in our last lesson video. But the difference was that with incomplete dominance in our last lesson video, the heterozygotes would create some kind of blended intermediate phenotype that did not really exist in either of the parents.

However, here with codominance, there's no blended intermediate phenotype. There are no pink flowers. Instead, with codominance, both phenotypes are going to be equally expressed. You see patches of white flower down below and you also see patches of red flower. The white and red portions of this flower are expressed equally in patches, and that is what codominance is all about. This is in heterozygotes that have one r₁ and one r₂ allele. This here helps you better distinguish between incomplete dominance and codominance. In our next lesson video, we're going to introduce one of the most common examples of codominance, which is blood type. So, I'll see you all in that video.

You cross two true breeding lines of petunia. One produces red flowers and the other white flowers. The F1 
offspring all show pink flowers. You conclude from this that:

• A. Red is dominant.
• B. White is dominant.
• C. Red is incompletely dominant.
• D. Red is incompletely recessive.
• E. None of the answers listed above are correct.

Answer: (C)

Codominance & Blood Type

In this video, we're going to talk about codominance as it relates to human blood type. It turns out that blood type in humans is a common and classic example of codominance. Of course, we introduced codominance in our last lesson video. Blood type in humans is determined by combinations of 3 possible alleles, which are IA, IB, or i. All combinations of these 3 alleles are possible for the genotype. The IA and IB alleles are actually the ones that are codominant to each other. That means that IA does not dominate over IB, and IB does not dominate over IA. So, they do not mask each other. Instead, they are going to both be equally expressed in patches whenever you have a heterozygote with both of these alleles. However, the lowercase i allele is recessive to the other two alleles, IA and IB.

If we take a look at our example down below at the ABO blood types, notice that we have a chart here with 3 columns. The first column represents the genotype. The second column represents the surface molecules that are found on the red blood cells themselves. The third and final column represents the final phenotype or the blood type that the individual would actually have. Notice that here in the first row of the genotype, what we have is IA, IA. So, 2 alleles for IA. Or we have one IA and one lowercase i. Again, recall that the lowercase i is recessive to IA, which means that only IA would be expressed. Either of these genotypes, either this one or this one, would lead to only A molecules on the surface of the red blood cell, similar to what you see over here. A red blood cell surrounded by these A molecules on the surface. And that, of course, would mean that this person would be Type A blood.

In the next row, what we have is IB, IB, or IB, lowercase i. Again, the lowercase i is recessive to IB, so only the IB is going to be expressed here in this genotype. But both of these genotypes here would lead to only B molecules on the surface of the red blood cell. So, it would look like this red blood cell here with these B molecules on the surface. And of course, that would mean that this individual would have Type B blood.

When an individual has one IA and one IB together, these two alleles are codominant to each other. This means that neither of them are going to dominate or mask the other. Instead, they are both going to dominate together. They're both going to be equally expressed. In that case, you get both A and B molecules equally expressed in patches on the surface of the red blood cell. Here we have the red blood cell, but notice that we have patches of A and B on the surface of this red blood cell in these patches. Neither of them dominate over the other; instead, they are both equally being expressed on the surface. This is a classic example of codominance. This person here would have blood type AB.

The final genotype is having two lowercase i's. An individual that has two lowercase i's is not going to have either molecule A or molecule B on the surface. Instead, they are going to have, a red blood cell that has neither A nor B on the surface. They are going to have type O blood. You can think of the O as almost like the naked look here of the type O blood without any surface molecules. This here concludes our introduction to codominance and blood type. We'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

If you look at a blood sample from a person who is heterozygous at the sickle cell locus you will see both normal circular red blood cells and sickle-shaped red blood cells. This is an example of

• A. multi-locus inheritance.
• B. complete dominance.
• C. incomplete dominance.
• D. codominance.
• E. linked genes.

Answer: (D)

A gene for the MN blood group has codominant alleles M and N. If both children in a family are of blood type M, which of the following situations is possible?

• A. Each parent is either M or MN.
• B. Each parent must be type M.
• C. Both children are heterozygous for this gene.
• D. Neither parent can have the N allele.

Answer: (A)

The number of different alleles for ABO blood types is _____ resulting in _____ different blood types.

• A. 3; 3.
• B. 4; 3.
• C. 6; 3.
• D. 3; 4.
• E. 2; 4.

Answer: (D)

Here’s what students ask on this topic:

What is the difference between incomplete dominance and codominance?

Incomplete dominance and codominance are two distinct patterns of inheritance. In incomplete dominance, heterozygous individuals exhibit a blended phenotype that is an intermediate of the two parent phenotypes. For example, crossing red and white flowers results in pink flowers. In codominance, both alleles are equally expressed in the phenotype. A classic example is human blood type, where alleles IA and IB coexist, resulting in an AB blood type. In summary, incomplete dominance results in a mixed phenotype, while codominance results in both traits being fully and equally expressed.

Can you provide an example of incomplete dominance?

An example of incomplete dominance is the inheritance of flower color in snapdragons. When a red-flowered plant (homozygous for the red allele, R1R1) is crossed with a white-flowered plant (homozygous for the white allele, R2R2), the offspring are heterozygous (R1R2) and exhibit pink flowers. The pink color is a blend of the red and white phenotypes, demonstrating incomplete dominance where neither allele is completely dominant over the other.

How does codominance manifest in human blood types?

Codominance in human blood types is exemplified by the ABO blood group system. The alleles IA and IB are codominant, meaning that both are equally expressed when present together. For instance, an individual with genotype IAIB will have both A and B antigens on the surface of their red blood cells, resulting in an AB blood type. Neither allele masks the other, and both traits are fully and equally expressed, which is characteristic of codominance.

What are some key characteristics of incomplete dominance?

Key characteristics of incomplete dominance include: 1) Heterozygous individuals exhibit a blended phenotype that is intermediate between the two parent phenotypes. 2) Neither allele is completely dominant over the other. 3) The resulting phenotype is a mix, such as pink flowers from red and white parents. 4) It differs from codominance, where both alleles are equally and fully expressed without blending.

What is a classic example of codominance in genetics?

A classic example of codominance in genetics is the ABO blood type system in humans. In this system, the IA and IB alleles are codominant. When an individual inherits both alleles (genotype IAIB), both A and B antigens are expressed on the surface of red blood cells, resulting in an AB blood type. This demonstrates codominance, where both alleles are equally and fully expressed in the phenotype.