Multiple Alleles

Many genes have multiple alleles: three or more alleles of the gene exist in the population. However, a diploid individual still carries only two of them, one on each homologous chromosome.The best-known example is the human ABO blood group, controlled by one gene with three alleles:• I^A codes for antigen A on red blood cells
• I^B codes for antigen B
• I^O codes for no antigenI^A and I^B are codominant with each other, and both are dominant to I^O, which is recessive. Three alleles give six possible genotypes but only four phenotypes:• Group A: I^AI^A or I^AI^O
• Group B: I^BI^B or I^BI^O
• Group AB: I^AI^B
• Group O: I^OI^OWorked example: a man who is I^AI^O (group A) and a woman who is I^BI^O (group B) have children. His gametes are I^A or I^O; hers are I^B or I^O. The offspring genotypes are I^AI^B, I^AI^O, I^BI^O and I^OI^O, so each child has a 1 in 4 chance of each of groups AB, A, B and O.Other examples of multiple alleles include coat colour in rabbits. In genetic diagrams, multiple alleles are usually written as one capital letter for the gene with superscripts for the alleles.