This lesson will examine the concept of codominance using blood types as an example.

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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on codominance. Today we are going to be investigating the concept of codominance. So codominance is when both alleles for a trait can be expressed together. So if we take a look at our example right here, normally what would happen with this type of genotype is that our big A here, because it is dominant, would be the trait that would be expressed in the individual.

And the little a, because it's recessive, would be masked. But in the example of codominance, both traits can actually be expressed together rather than one dominating over another. So a common example of codominance is with blood type. So alleles that are present for blood type are A, B, and O.

So we actually have three alleles for blood type-- one, two, three. Now, many traits actually only have two alleles for the trait. But sometimes there can be three alleles that can represent a trait. And we call that the multiple allele system when a gene has three or more alleles. So blood type is an example of a multiple allele system because we have three alleles for that trait.

And with blood type, A and B are considered codominant. So this means that they can be expressed together, and one is not dominant over the other. But O is actually considered a recessive gene. So we're going to take a look here at an example. So as I mentioned, A and B are codominant and can be expressed together, so that's the part we want to keep in mind.

So we're going to look at the genotype and phenotype of different combinations of these alleles. OK, so let's say we have a genotype of AA or AO. Now remember for each trait, we always have two alleles, one donated from our mother and one from our father.

So even though there are three possible alleles for blood type, we're only going to have two present in our genes, one from mom and one from dad. So if we have the genotype big A big A or big A O, our phenotype or the blood type that's going to be a result of those genes is that we're going to have type A blood.

So remember always recessive. So any other genetic problems that you've done in the past, if you're familiar with recessive genes and how those work, if a recessive gene is in the presence of a dominant gene, the recessive gene is masked. So in this case, A is dominant over recessive, so we're going to have a phenotype of type A blood.

Let's say that the genotype is BB or BO. Again, O is recessive, so it's going to be masked by this B allele. So we're going to end up with a phenotype of type B blood as a result of these genotypes. All right, so here's where we get into the codominance.

Let's say that we inherit an A allele from our mom and a B allele from our dad. So as we mentioned, A and B are codominant and can be expressed together. So the A does not mask the B, and the B does not mask the A. They can be codominant.

So our phenotype is actually going to be type AB blood. They're both expressed. And then our last genotype that could be possible here is OO. And then in that case, if you have two recessive alleles together, then they can actually be expressed, and our blood type would be type O.

So this lesson has been an overview on codominance and using blood type as an example.

Terms to Know

When two alleles express themselves vs. one dominating the other; an example would be A.B.O. blood groups.

Multiple Allele System

A gene that has three or more alleles or alternative sets of genes, again an example would be the A.B.O. blood group.