Multiple Alleles: The ABO Blood Group

Though we typically teach about genetics using the simplest possible genetic situations, more complicated relationships exist. In fact, the more complicated gene interactions are probably much more common than the simple ones which are so easy to figure out.

Among the variety of more complex situations, one of the most common is multiple alleles. Typically, we teach with genes for which only two alleles are known, but many genes have more than two different alleles--thus, "multiple" alleles.

One such gene which is of great interest to humans is the ABO blood group gene. This particular gene has three alleles, rather than two. Of course, each of us has only two sets of chromosomes, so any one individual has only two of these alleles at once. But the presence of three different alleles means that there are six possible genotypes, rather than the three possible for the more familiar two-allele situation.

For the ABO gene, the three alleles are the IA, IB and i alleles. We typically call these alleles "A," "B," and "O," but of course our rules for assigning symbols to alleles demand that all three be represented by some version of the same symbol. In this case, that common symbol is the letter "I," which stands for "immunoglobin."

By now, the concept of dominance should be familiar to you. Of course, things get a bit more complicated when there are three alleles instead of just two. As the symbols above should suggest, the i allele (the "O" allele) is recessive to both the IA and IB alleles (the "A" and "B" alleles). The IA and IB show co-dominance. This means that in an individual who is heterozygous for these two alleles, the phenotypes of both alleles are completely expressed, thus producing blood type AB.

Thus we have the following:

Type O
Type A
Type B
Type AB

Clearly, the additional genotypes produced by having three alleles rather than just two make for much more fun when it comes to figuring out genetic interactions between two people :^)

Other Blood Groups:

The ABO blood group isn't the only aspect of our blood type chemistry. There are about a dozen known genes which affect "blood type" activity. Because the ABO blood type has been known the longest, and has such a powerful effect, it is the one we focus on. The genes for the others work in similar fashions, though certainly not all have three alleles.

The other aspect of blood type which is of most interest to us is the Rh factor. Genetically, this is much simpler than the ABo system. It has only two alleles, one dominant (Rh-positive) and one recessive (Rh-negative). However, this facet of our blood type has some serious implications when we reproduce, and so has come under pretty heavy scrutiny. You can explore more about this part of your blood type by reading the Rh Disease essay.

Just what is blood type, anyway?

Our blood type is part of the marvelous protective machinery in our bodies called the Immune System.

Our immune system's task is to identify and destroy biological materials which are foreign to our own bodies. In order to perform this task, of course, your immune system needs to know how to distinguish between what belongs to you and what is foreign. This isn't the easiest of tasks, as you are made out of precisely the same materials that would compose any living invader. So a vital part of your immune system is the tagging of your own cells and tissues, so your antibodies won't destroy your own cells.

Blood type is part of this much larger self-tagging system aspect of your immune system. "Blood Type A" actually means "blood cells tagged with antigen A." Antigen A is a specific protein marker found on the surfaces of all Type A blood cells. And the task of the IA allele is to cause the creation and attachment of this specific kind of antigen. The IB allele causes the creation and attachment of a different protein marker, the B antigen.

Knowing this about blood type can also explain to you why the IA and B alleles are co-dominant, and why the i allele is recessive. The IAIB genotype results in both A and B antigens on the cell surfaces. The i allele causes no antigen to be produced, and is thus a "silent" allele.

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Updated 25 September 2004