A human gene that typically exhibits more than one allele is the gene encoding an enzyme that adds sugar molecules to lipids on the surface of red blood cells.

These sugars act as recognition markers for cells in the immune system and are called cell surface antigens. The gene that encodes the enzyme is designated I and has three common alleles: B, whose product adds the sugar galactose; A, whose product adds galactosamine; and O, which codes for a protein that dos not add a sugar.

Different combinations of the three I gene alleles occur in different individuals because each person possesses two copies of the chromosome bearing the I gene and may be homozygous for any allele or heterozygous for any two. An individual heterozygous for the A and B alleles produces both forms of the enzyme and adds both galactose and galactosamine to the surface of red blod cells.

Because both alleles are expressed simultaneously in hetrozygotes, the A and B alleles are codominant. Both A and B are dominant over the O allele because both A or B alleles lead to sugar addition and the O allele does not. The different combinations of the three alleles produce four different phenotypes:

1. Type A individuals add only galactosamine and are either AA homozygotes or AO heterozygotes.

2. Type B individuals add only galactose and are either BB homozygotes or BO heterozygotes.

3. Type AB individuals add both sugars and are, as we have seen, AB heterozygotes.

4. Type O individuals add neither sugar and are OO homozygotes.

The four different cell-surface phenotypes listed above are called the ABO blood groups or, less comonly, the Landsteiner blood groups, after the man who firs described them. As Landsteiner noted, a person's immune system can distinguish between these four phenotypes. If a type A individual receives a transfusion of type B blood, the recipient's immune system will recognize that the type B blood cells possess a "foreign" antigen (galactose) and attack the donated blood cels, causing the cells to clump or agglutinate.

The same will happen if the donated blood is type AB. However, if the donated blood is type O, no attack will occur, as there are no galactose antigens on the surface of blood cells produced by the type O donor. In general, any individual's immune system will tolerate a transfusion of type O blood. Because neither galactose nor galactosamine is foreign to type AB individuals whose red blood cells have both sugars, those individuals may receive any type of blood.

Some of the ABO blood group phenotypes are more common than others in human populations. In general, type O individuals are the most common, and type AB individuals the least common. However, there is a wide variation in ABO group frequencies in different populations. Among indigenous North American, the frequency of type A individuals is 31%, while among indigenous South Americans, it is only 4%