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 typeAB.
If 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%. Since AA, AO and OA are blood type A, and OO is blood type O, thus their child has 6.25% chance to be blood type O and 93.75% chance to be blood type A.
The +/- is called the rhesus factor, with + being dominant, and - being recessive. So if both parents are -, the kids are always -, otherwise the kids might be + or -.
Child Blood Type Estimate Table: Father's Blood TypeABABOMother's Blood TypeAA/OA/B/AB/OA/B/ABA/OBA/B/AB/OB/OA/B/ABB/OABA/B/ABA/B/ABA/B/
