Two A-type blood types can give birth to an O-type child. Inheritance of the ABO blood group system in general is determined by a single gene. the ABO gene is located on the long arm of human chromosome 9 and has three major alleles i(A), i(B) and i(O). Since the human chromosome is a diploid, a person can usually only have two of these, from each parent. The type of these two alleles determines the human blood type. Secondly, it is important to realize that both I(A) and I(B) are dominant to i(O), e.g., if a person’s genotype is I(A)I(O), his blood type is A blood; if a person’s genotype is I(B)I(O), his blood type is B blood. Therefore, if both parents’ genotypes are I(A)I(O), each parent has one allele, and at this time, there are three possible genotypes for the child, i.e., I(A)I(A) child has blood type A, I(A)I(O) child has blood type A, and I(O)I(O) child has blood type O. Therefore, there is a one-third possibility that the child will have type O blood. In conclusion, two parents with blood type A can have a child with blood type O with a one-third probability.