So far, genes have had only two alleles. Some genes have more than two alleles in the population. These are called multiple alleles.Each diploid individual still carries only two alleles of the gene, one on each of a pair of homologous chromosomes. The extra alleles exist in other members of the population.The ABO blood group gene
The gene I controls an antigen on the surface of red blood cells. It has three alleles:
• IA gives antigen A
• IB gives antigen B
• IO gives neither antigenIA and IB are codominant with each other, and both are dominant to IO, which is recessive. This gives six genotypes but only four phenotypes:IAIA or IAIO = group A
IBIB or IBIO = group B
IAIB = group AB
IOIO = group OMultiple alleles can also form a dominance hierarchy. Coat colour in rabbits has four alleles, with agouti dominant to chinchilla, chinchilla dominant to Himalayan, and Himalayan dominant to albino.Worked example
With n alleles, the number of possible genotypes is n(n + 1) ÷ 2. For the four rabbit alleles:number of genotypes = 4 × 5 ÷ 2 = 10 For the three ABO alleles the same formula gives 3 × 4 ÷ 2 = 6 genotypes, matching the list above.
The gene I controls an antigen on the surface of red blood cells. It has three alleles:
• IA gives antigen A
• IB gives antigen B
• IO gives neither antigenIA and IB are codominant with each other, and both are dominant to IO, which is recessive. This gives six genotypes but only four phenotypes:IAIA or IAIO = group A
IBIB or IBIO = group B
IAIB = group AB
IOIO = group OMultiple alleles can also form a dominance hierarchy. Coat colour in rabbits has four alleles, with agouti dominant to chinchilla, chinchilla dominant to Himalayan, and Himalayan dominant to albino.Worked example
With n alleles, the number of possible genotypes is n(n + 1) ÷ 2. For the four rabbit alleles: