With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population.
Codominance: When both alleles of a pair are fully expressed in a heterozygote, the genes and trait are said to be codominant. Example: A person of blood group AB is an example of codominance where allele IA for A-type blood is codominant with its allele IB for B-type blood. The heterozygote (IAIB) expresses the characteristics of both A and B antigens.
Multiple allelism: It is a phenomenon which occurs when more than two alleles exist at a given locus of a chromosome. In a given individual, only two of these alleles occur, one derived from each parent. Example: The ABO blood typing in humans is an example of multiple allelism where alleles IA, IB and i produce the four phenotypes (A, B, AB and O) of blood groups.