Question

With the help of one example explain the phenomenon of codominance and multiple allelism in human population.

Answer 1

Codominance is a phenomenon in which both the alleles in an heterozygous condition are expressed equally and manifest themselves in the phenotype, resulting in a condition in which the phenotype is neither dominant nor recessive. Multiple allelism is a condition in which not two but rather multiple alleles are present for a single phenotype i.e a gene exists in multiple allelic forms. E.g: In ABO blood grouping in humans three alleles are seen I${}^A$, I${}^B$ and I${}^O$, that result in the different genotypes. I${}^A$ and I${}^B$ result in antigens A and B that show codominance and both of them are expressed equally when present together. Hence in blood group AB where both the alleles are present, both antigens are seen on the surface of the blood cells. Also the three allele I${}^O$ does not result in any antigen and the three alleles together result in the genotypes:

I${}^A$I${}^A$ : Blood group A

I${}^A$I${}^O$ : Blood group A

I${}^B$I${}^B$ : Blood group B

I${}^B$I${}^O$ : Blood group B

I${}^A$I${}^B$ : Blood group AB

I${}^O$I${}^O$ : Blood group O

So, ABO blood grouping can be considered an example of both multiple allelism and codominance.