Can two sisters have different blood groups?
Absolutely! Those are the only two possibilities. The blood groups are controlled by one gene that has three different “flavors”. Each person has two copies of the gene. So with two copies of the A “flavor” you will be type A. With two copies of the B flavor you will be B. These are dominant flavors, so all you need is one. If you have one copy of B and one copy of A you will be blood type AB. The third flavor is recessive and you need two copies of the recessive flavor to be blood type O. For one child to be B and the other O, the father has to have one copy of B and one recessive copy. The + sign is also called the Rh factor and it comes from a bunch of genes. Since mom and dad are +, then it’s no surprise that both kids are positive.
An important issue is the difference between phenotype – how things appear and genotype – the controlling factors or genes involved. The father appears as blood type B but his controlling factors are the genes for blood type B and blood type O, with O being recessive or “weaker”. The mother is blood group O she has only O genes. It is possible for one of the children to get the B from the father and the O from the mother and have the genes B.O and show blood group B. The other child got O from the father and O from the mother and is blood group O.
