Question

A couple sired four children with blood types A, B, AB, and O. What is the genotype of the two parents?

Answer 1

Recall that the human ABO gene on chromosome 9 has three common variants of the gene. Different variants are called alleles. The A allele encodes N-acetylaminogalactosyltransferase and this enzyme makes the A antigen that confers blood type A. The B allele encodes a variant enzyme that makes B antigen and gives rise to blood type B. The O allele encodes a defective enzyme that doesn't make either antigen. In the absence of both A antigen and B antigen your blood type will be O.

Everyone has two copies of chromosome 9 so you have two ABO genes. If both of them are the A alleles then your genotype (genetic makeup) will be AA. Your blood type will be A. If one of your ABO genes is the A allele and the other is the O allele then your genotype will be AO. In this case your blood type will be A as well since there are only three possibilities: (1) you have A antigen, (2) you have B antigen, (3) you have no antigen (you can have both antigens, see below). As long as you have one A allele, you will produce the A antigen on the surface of your blood cells.

Now let's look at the possibilities for inheritance of blood types. If someone has blood type A then their genotype might be AA or AO.