Predict offspring ABO and Rh blood types from parents.
Blood type is determined by the ABO gene on chromosome 9. There are three alleles: Iᴬ (A), Iᴮ (B), and i (O). A and B are codominant; both are dominant over O. Possible genotypes: AA or AO → type A, BB or BO → type B, AB → type AB, OO → type O. Since we can't distinguish AA from AO phenotypically, predictions from phenotype alone give ranges. Two type O parents can only have type O children. A type AB parent can never have a type O child. Blood typing is important for transfusions (universal donor: O−, universal recipient: AB+), paternity testing, and forensics. The Rh factor (+ or −) is separately inherited on chromosome 1.