1.6 multiple alleles
If a gene controlling a characteristic has three or more alleles, the alleles are called multiple alleles. In any one individual, at most two alleles can be present.
An example of multiple alleles is provided by the alleles of the gene controlling the ABO blood group system in humans.
We all possess 2 ABO blood type alleles, one from each parent. The gene for the ABO blood system is conventionally represented by the symbol l. The gene has three alleles. I^A, I^B and I^O are the alleles which code for the A, B and O blood groups respectively.
Table 1.3 shows the possible allelic combinations, the resulting phenotypes and a description of the mode of inheritance.
The alleles I^A and I^B show equal dominance with respect to each other, i.e. they are co-dominant, but each is dominant to I^O. Only two of the three alleles can be present in an individual. Table 1.3 show the possible genotypes one can have. For blood group AB, both type A and B antigens are present on the red blood cell membrane.