Question

What type of genetic inheritance determines the ABO blood group system? What are the relations of dominance among the involved alleles?

Answer 1

The inheritance of the ABO blood system is a form of multiple alleles inheritance. There are three alleles involved, I^A, I^B and i which combine in pairs to form the genotypes.

Concerning dominance, the allele i is recessive in relation to the alleles I^A and I^B. Between I^A and I^B, however, a lack of dominance is established and the heterozygous pair (I^AI^B) has its own distinct phenotype.

Humans can have red blood cells that are of type A (I^A), type B (I^B), or type O (i). These three different alleles can be combined in different ways following Mendel's Laws of Inheritance. The resulting genotypes make either type A, type B, type AB, or type O blood.

Type A blood is a combination of either two A alleles (I^A I^A) or one A allele and one O allele (I^Ai). Similarly, type B blood is coded for by either two B alleles (I^B I^B) or one B allele and one O allele (I^Bi). Type O blood can only be obtained with two recessive O alleles (ii). These are all examples of simple or complete dominance.

Type AB blood is an example of co-dominance. The A allele and the B allele are equal in their dominance and will be expressed equally if they are paired together into the genotype I^A I^B. Neither the A allele or the B allele is dominant over each other, so each type is expressed equally in the phenotype giving the human an AB blood type.