Blood Type Genetic Basis: In genetics, blood type gene has two alleles, each allele has genotype A, B or O. The A and B are dominant, and O is recessive.So allele A combined with allele O is type A. Similarly, BO is type B, AA is type A, BB is type B, OO is type O, and AB is type AB.f both parents have type A blood, then the alleles could be AA or AO, thus the allele A frequency is 75%, allele O frequency is 25% for both parents. So the chance of alleles OO is 25% × 25% = 6.25%, alleles AA is 75% × 75% = 56.25%, alleles AO is 75% × 25% = 18.75%, alleles OA is 25% × 75% = 18.75%.