Multiple Alleles

Multiple alleles means that a gene has more than two alleles in the population. However, each diploid individual can only carry two of them, one on each chromosome of a homologous pair, because it has only two copies of the gene locus.Multiple alleles arise because different mutations of the same gene produce several versions of the gene. They often show a hierarchy of dominance, and some pairs may be codominant.Example: the ABO blood group gene. The gene I codes for an enzyme that adds sugars to an antigen on the surface of red blood cells. There are three alleles:• I^A: produces antigen A
• I^B: produces antigen B
• I^O: produces neither antigenI^A and I^B are codominant with each other, and both are dominant to I^O, which is recessive. This gives six genotypes and four phenotypes (blood groups A, B, AB and O).Example: rabbit coat colour. The gene C has four alleles in a dominance series: C (agouti, full colour) > c^ch (chinchilla) > c^h (Himalayan) > c (albino).The number of possible genotypes for n alleles is n(n + 1) ÷ 2. Worked example: with four rabbit coat alleles, there are 4 × 5 ÷ 2 = 10 genotypes, but only four phenotypes because of the dominance series.