SAT Biology Subject Test Practice Questions

Answer: C, Recessive: A child can only have O blood type if both parents have O blood type or are heterozygous for A or B blood type.

Key takeaway: Blood type A and blood type B are co-dominant in humans. If a child receives the antigen for blood type A from one parent and B from the other, the child’s blood type is no either A or B, but a combined AB. In this system, a child can only have blood type O if it does not receive a blood type antigen from either parent. This is only possible if the parents are homozygous for O blood type or heterozygous for A or B blood type.