Q.What is co-dominance ? Give an example.

Co-dominance is the simultaneous, full expression of both alleles in a heterozygote, illustrated by the ABO blood group system.

Co-dominance occurs when two different alleles of a gene are both fully expressed in the heterozygous condition, so that the phenotype shows the distinct effects of both alleles side by side, rather than one masking the other (as in simple dominance) or the two blending into an intermediate phenotype (as in incomplete dominance).

Example -- ABO blood grouping in humans: the gene controlling ABO blood type has three alleles -- I-A, I-B and i. I-A and I-B are each dominant over i, but I-A and I-B are co-dominant to each other. A person with genotype I-A I-B therefore expresses both the A antigen (specified by I-A) and the B antigen (specified by I-B) on the surface of their red blood cells simultaneously, resulting in blood group AB -- both antigens are fully present and detectable, neither being suppressed by the other.

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