How are ABO blood groups genetically transmitted?

The ABO blood group is an example of codominance and multiple alleles. Unlike other characteristics which are determined by 2 alleles, blood group is determined by 3 alleles. The allele IA corresponds to blood group A (genotype IA IA ) and the allele IB corresponds to blood group B (genotype IB IB) . Both these are dominant so if IA and IBare present together they form blood group AB (genotype IAIB ). The allele i is recessive to both IA and IB and hence if you have genotype IAi you will have blood group A and if you have genotype IAi you will have blood group A and if you have the genotype IB i you will have blood group B. However, if you have the genotype ii then you are homozygous for i, and you will have blood group O.

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