7.10: What’s your blood type? Some genes have more than two alleles.

Do you know your blood type? It can be O, A, B, or AB. Each of these different blood types (also called blood groups) indicates something about the physical characteristics of your red blood cells and has implications for blood transfusions—both giving and receiving blood. The blood groups are also interesting from a genetic perspective, because they illustrate a case of multiple allelism, in which a single gene has more than two alleles. Each individual still carries only two alleles—one from the mother and one from the father. But if you survey all of the alleles present in the population, you will find more than just two alleles.

Inheritance of the ABO blood groups provides the simplest example of multiple allelism, because there are only three alleles. We can call these alleles I^A, I^B, and i. The I^A and I^B alleles are both completely dominant to i, so individuals are considered to have blood type A whether they have the genotype I^A I^A or I^A i (FIGURE 7-20). Similarly, an individual with the genotype I^B I^B or I^B i is considered to have blood type B. If you carry two copies of the i allele, you have blood type O. The I^A and I^B alleles are codominant with each other, so the genotype I^AI^B gives rise to blood type AB. Consequently, with these three alleles in the population, individuals can be one of four different blood types: A, B, AB, or O.

What are the phenotypes of these alleles? An individual’s blood-type alleles carry instructions that direct construction of a specific set of chemicals, called antigens, that protrude from every red blood cell. These antigens are molecules (chiefly carbohydrates bound to protein) that jut from the surface of a cell and can “turn on” a body’s defenses against foreign invaders. The I^A allele directs the production of A antigens all over the surface of red blood cells. Similarly, the I^B allele directs the production of B antigens on all red blood cells. The i allele does not code for the A antigen or the B antigen.

This means that individuals with blood type AB have red blood cells with both A and B antigens, while individuals with blood type O have red blood cells that have neither A nor B antigens on their surface (FIGURE 7-21).

Antigens on red blood cells play a role in the body’s disease-fighting immune system. Antigens are like signposts, telling the immune system whether a cell belongs in the body or not. If a red blood cell with the wrong antigens enters your bloodstream, your immune system recognizes it as a foreign invader and destroys it. Such an attack is initiated by molecules in the bloodstream called antibodies, which attack only foreign antigens. Individuals with only A antigens on their red blood cells produce antibodies that attack B antigens. If these cells encounter a red blood cell with B antigens, they attack it. Such an immune response can lead to destruction of red blood cells, low blood pressure, and even death. Under normal circumstances, antibodies do not encounter a red blood cell with foreign antigens. Such an event may occur, however, if red blood cells with foreign antigens are accidentally injected into the person’s bloodstream in a transfusion.

Individuals with only B antigens on their red blood cells produce antibodies that attack A antigens. Individuals with blood type O, who have neither A nor B antigens on their red blood cells, produce antibodies that attack both A and B antigens. Individuals with blood type AB don’t produce either type of antibody (or else they would have antibodies that attacked their own blood cells). From this information, we can deduce which blood types can be used in transfusions. Individuals with blood type O are universal donors, because their red blood cells have no A or B antigens and so do not trigger a reaction from either type of antibody. And individuals with blood type AB are universal recipients, because they do not produce antibodies to either the A or B antigen. This is shown in FIGURE 7-22.

Another marker on the surface of red blood cells is the Rh blood group marker. (Note that the Rh blood group is not an example of multiple allelism. A single gene with just two alleles determines the presence of the Rh marker. However, like the ABO blood groups, the Rh marker restricts the type of blood a person can receive in a transfusion.) Individuals who possess red blood cells that carry the Rh cell surface marker have one or two copies of the dominant Rh marker allele, and they are said to be “Rh-positive.” This “positive” (or “+”) is noted along with their ABO blood type, as in “O-positive” or “B-negative.” Individuals who have two copies of the recessive allele for this gene do not have any Rh markers, and they are described as “negative,” as in “O-negative” or “A-negative.” If, during a blood transfusion, individuals who are Rh-negative are exposed to Rh-positive blood, their immune system attacks the Rh antigens as foreign invaders—an immune response that can vary from mild, which passes unnoticed, to severe, which can lead to death.

Beyond the ABO marker groups, there are many, many genes with multiple alleles—a dozen or even more alleles in some cases. In fact, one gene for eye color in fruit flies has more than 1,000 different alleles!