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Codominance & Genes with Multiple Alleles

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  • 0:00 More than Two Alleles
  • 0:36 Codominance in Human…
  • 2:53 Multiple Alleles Have…
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Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Some genes have more than two alleles, which increases the possible genetic outcomes for offspring. And as we'll see in this lesson, sometimes those alleles work together to produce a codominant phenotype.

More Than Two Alleles

If you've ever donated blood, you have surely been asked your blood type. Are you A, B, AB, or O? Did you ever stop to think about what this means? Or the fact that these are alleles on your genes? When we talk about genes and alleles, which are just different versions of genes, we generally refer to genes with only two alleles. For example, red or white flower color.

But some genes can have more than two alleles, which makes for some interesting genetic inheritance patterns. Let's look at this concept more closely using human blood type as an example.

Codominance in Human Blood Types

In humans, there are three different alleles that can create four different phenotypes, or traits that are expressed. The alleles are A, B, and O, and the phenotypes are A, B, AB, and O. The letters correspond to the carbohydrate found on the surface of that person's red blood cells. So if a person's red blood cells are coated with carbohydrate A, we say that person has blood type A. If their red blood cells are coated with carbohydrate B, then they have blood type B. But, they could also have red blood cells that contain both carbohydrates, in which case their blood type is AB. And finally, they may have red blood cells that are coated with neither carbohydrate, in which case their blood type is O.

Since you get one blood type allele from each parent, there are several possible combinations that you could inherit. If you got two A alleles (one from each parent), then you would have a phenotype of blood type A, as well as genotype, or genetic makeup AA. If you got two B alleles, then you would have blood type B and genotype BB.

But here's where things get a bit complicated, because both the A and B alleles are dominant, meaning that they determine an organism's phenotype. For example, if blonde hair is dominant and brown hair is recessive, then it only takes one blonde hair allele for you to have blonde hair. You could very well have a brown hair allele too, but it would only show up in your genotype, not your phenotype.

Because both A and B are dominant, if you get one A allele from one parent and one B allele from the other, then your blood type and genotype would be AB. This is an example of codominance, or when both alleles are expressed in the phenotype.

But what's really cool is that the O allele is recessive, meaning that it is genetic information that does not affect an organism's phenotype unless you have two of those alleles. If you carry an O allele, you can still pass it on to your children, but if you have just one O allele, it won't affect your blood type because A and B are dominant. So if your genotype is AO, then you have blood type A, and if your genotype is BO, then you have blood type B. The only way to have blood type O is to have genotype OO, because there is no dominant allele to mask the recessive allele.

Multiple Alleles Have Genetic Benefits

You might be wondering why organisms would have codominance and multiple alleles. Well, there are some advantages to having such a variety of combinations and phenotypes. The reason it is so important to know your blood type is because it determines what type or types of blood you can receive and what others can receive from you.

Your body doesn't like foreign invaders, because these can be harmful things that hurt or kill you. So if you have blood type A and you receive a donation of blood type B, your body will produce certain proteins called antibodies. These proteins are designed to identify and destroy pathogens and viruses in the body, and when a mismatched blood type appears, those antibodies can't tell the difference. They bind to those unknown carbohydrates and kill the blood cells, which is exactly what you don't want to happen to your new blood!

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