 So let's think a little bit about genetics as it's related to the human ABO blood type. This is a really interesting situation because it exhibits both co-dominance and multiple alleles. In the situation of co-dominance, when there are two different alleles, both are expressed equally. This is also a situation in which we have multiple alleles for one particular gene locus. In the human ABO group, there are three alleles that may be involved with determining one's blood type. Of course, any individual only has two of each allele, but there are three in the population. One of these we refer to as the IA allele. IA is just a standard convention, but you could use a different system if you wanted to. But the IA allele codes for antigen A, and I've just represented antigen A on this red blood cell. This circle is representing a red blood cell, and the little triangles are just representing the A antigen that's found on the surface of the red blood cells. The IB allele codes for antigen B, and I've represented antigen B as a little semicircle on this circle, which represents a red blood cell. Then we have the little I allele, and the little I allele doesn't code for an antigen like antigen A or antigen B, and so I've drawn a red blood cell here that doesn't have an antigen displayed on its surface. Three different alleles are involved with determining the human ABO blood type. So I've created a little chart here that lists the different human blood types, and it shows the genotypes that are available. Blood type would be the phenotype or the physical characteristic in this situation, and then the genotypes are which combination of alleles the individual might have. So folks with type A blood could be homozygous IAA, or they could be IA little I. In both situations they have the A antigen on their red blood cells. People that have type B blood are either IB, IB, or IB little I. In both situations they have the B antigen on the surface of their red blood cells. Then there are folks that have type AB blood. In order to have type AB blood, one must have both the IA allele and the IB allele. And so type AB blood would look like this. The IA allele would cause the A antigen to be produced and displayed on the surface of the red blood cell membrane, and the IB allele would cause the B antigen to be produced and displayed on the surface of the red blood cell. So they have both antigens on their red blood cells. Then we have the type O blood type, and the only possible way to get type O blood is for one to have a homozygous little I situation. So little I results in type O blood, and this is what a type O blood cell would look like. No antigens displayed on the surface of those membranes. So human ABO blood type is an example of co-dominance and multiple alleles.