Exceptions to Simple Dominance: Codominance and Incomplete Dominance

Alright, so Adrian is feeling pretty good now. He's already determined the genetic basis of two flying hamster traits - coat color and ear size - and he found that both traits are controlled by a single gene in which one trait is dominant over a recessive trait. With a little help from us, he's also rediscovered Mendel's laws, which taught him that alleles on homologous chromosomes segregate into separate gametes (Law of Segregation) and also that alleles of one gene segregate independently of the alleles of another gene (Law of Independent Assortment).

Puzzling Tail Color Results

So during his latest trip to collect new and exciting strains of flying hamsters, he notices flying hamsters with different tail colors, and he decides that that's the next genetic trait that he's going to try to study. Like he's always done, he goes ahead and he establishes some true-breeding strains and begins his experimental crosses.

He starts out first with some strains of brown hamsters with blue tails and another strain of brown hamsters with yellow tails and mates those guys together. But when he mates them together, he gets green-tailed hamsters! So he's kind of thinking, 'What the heck is going on?'

ABO Human Blood Type

So Adrian decides that he's going to walk down the hall and consult with his human geneticist friend, Ben, to see if he can help him explain these really strange results.

Adrian explains to Ben that he has these blue-tailed hamsters and these yellow-tailed hamsters that he mates together and somehow, instead of getting either one of those phenotypes, he gets an entirely different phenotype; he gets green-tailed hamsters.

Ben thinks about this for a little bit, and then he tells Adrian that inheritance isn't always as easy as he's been observing so far. He tells him to listen to this tale of the ABO blood type gene and how it defies some of the ideas that Mendel has proposed.

Multiple Alleles

Have you ever wondered what your blood type means or why you can only receive blood from certain people? Well, it's a more complex trait than we've been considering so far. For starters, there's more than one allele; there are multiple alleles for this gene. One allele is called the A allele and it helps produce a sugar that is found on the outside of red blood cells. The second allele, allele B, also helps produce an extracellular sugar, which is slightly different but also expressed on the outside of red blood cells. The third allele, the o allele, produces no extracellular sugar, so you end up with a red blood cell with no extracellular sugars on the outside.

Recall in our previous studies, we wanted to establish which allele is dominant. So with three different alleles, how can we figure out which allele is dominant? If I'm trying to figure out which allele is dominant, I'm going to have to check the progeny of homozygous individuals.

Codominance

The children of an AA and oo individual will have an Ao genotype. If I look at red blood cells from those children under the microscope, I can only see A sugars on the outside of the cells. So, we can say that the A phenotype is dominant over the o phenotype, which kind of makes sense because the o phenotype is really no extracellular sugars.

If someone who has a BB genotype and someone who has an oo genotype have children, all of their children would have a Bo genotype. If we were to look at the red blood cells from those children, we would see that the sugars expressed on the outside of the red blood cells are all of the B type. Again, we can then say that the B phenotype is dominant over the o phenotype.

That all sounds pretty simple and easy to understand, right? But, we still have to consider children of an AA individual and a BB individual. We can predict that all of their children will have an AB genotype.

Now, when we look at the red blood cells of these children under the microscope, we see both the A sugars and the B sugars simultaneously on the outside of the same red blood cells. This is definitely a lot different than what we've seen before in all of our examples of simple dominance, where one phenotype is observed and the other phenotype is masked in a heterozygote. In this case, both phenotypes can be observed in the heterozygote at the same time, so we call this codominance; both of the alleles are dominant.

These aspects of the ABO blood type explain why we can only accept blood from certain people.

Universal Blood Donors and Recipients

The immune system recognizes foreign particles, called antigens, as things that aren't normally in your body and attacks them. Rather than refer to the sugars, scientists and doctors usually refer to them as A antigens or B antigens.

If you're an individual that's either AA or Ao, you're going to have these A antigens on the outside of your red blood cells. But, that means that there are no B antigens anywhere in your body. So, if you received blood from a BB individual or a Bo individual, the immune system would recognize the red blood cells with B antigens as foreign bodies, and the rejected red blood cells would be attacked and destroyed. Similarly, if you're someone with a B blood type, you have a BB or Bo genotype, and your body is used to seeing the B antigens in the blood stream. As soon as it sees red blood cells with A antigens, it's going to attack those red blood cells, and you're not going to get the benefit of that blood transfusion.

The interesting part though is if you're an individual with an oo or an AB genotype.

If you're an oo individual, you don't have any antigens on the outside of your red blood cells. So, that means that you can only receive blood from someone else that's an o-type because any blood that has an A antigen or B antigen is going to trigger an immune response and rejection of the blood.

Contrast that to someone who has an AB blood type. That person's immune system is used to seeing both the A and the B antigens. So this person could receive not only AB-type blood but also A- or B-type blood. That means that a person with an AB blood type can receive blood from someone with an AA, Ao, BB or Bo genotype because neither the A nor B antigens will elicit an immune response.

People with an AB blood type as well as those with an A or B blood type can all can receive blood from an o-type person because that person produces red blood cells with no antigens that can react with the immune system. We already said that someone with o-type blood can receive blood from another o-type person, so that means a person who is o-type can donate blood to anyone and, therefore, is called a universal donor. Similarly, since people who are AB-type can receive blood of any type, they are called universal recipients.

Incomplete Dominance

So now that Adrian has learned about ABO blood type, he rushes back to the lab and decides he's going to try to figure out what's going on with this hamster tail color gene. Let's see if we can help Adrian develop a hypothesis for what's going on here.

Let's represent our blue allele with 'B' and our yellow allele with 'Y'. So if we have true-breeding strains here in our P generation, that means that one of the parents is 'BB' and one of the parents is 'YY'. That necessarily means that all of the hamster progeny have a 'BY' genotype.

Since we know that these hamsters have a green tail instead of a blue or yellow tail, what if we hypothesize that these traits aren't all or none, that we're getting some sort of phenotypic blending of the traits, that the blue or the yellow are combining to produce green?

So how can we determine if that is the case? What if we consider a Punnet Square again?

Let's write in the genotypes if we were to mate F1 green-tailed hamsters together. If we have 'BY' and 'BY' as our parent genotypes, we would produce 'BB', two 'BY' and one 'YY'. If I see at the end a ratio of 1:2:1 between blue, green, and yellow hamsters, we verified our hypothesis.

Since we do observe these phenotypic ratios, we can conclude that the flying hamster tail color gene that we've been studying follows what's called incomplete dominance. That means is that the heterozygote exhibits a phenotype that is intermediate between the phenotype of the two homozygotes. In this case, the homozygotes have either blue or yellow tails, while the heterozygotes have green tails.

Lesson Summary

In summary, we've learned that many traits are defined by multiple alleles. For instance, we saw that there are three alleles at the human ABO blood type locus. Codominance is a state in which the phenotypes of two different alleles are simultaneously expressed in a single heterozygote. Incomplete dominance is a state in which the phenotype of the heterozygote is intermediate between those of the homozygotes.