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Well it looks like you have two different things going on here. If the father is AA and the child is As, then the mother contributed the s gene; we can only give her genotype as "s?" without additional information. For the blood groups, the blood type ( A, B, AB, or O) is a separate gene than the Rh factor, which gives you the + or - part of the blood type.
Let's look at the blood type first. A and B are two proteins that can occur on the outer membranes of the blood cells, they are part of the body's mechanism of identifying self versus intruder for the immune system. A person with type A blood has the A protein on his/her cells, a person with type B blood has the B protein, and a person with type AB has both the A and B proteins. A type O person has neither of those proteins. The O blood type is recessive, so in order for the father to be type O he has to have 2 O genes. The mother is type B, so she has at least one B gene, and we can't be sure about the other - she could be BB or BO. Their son's genotype is BO, because he got a B gene from the mother and an O gene from the father.
The Rh factor is yet another surface protein. If you are Rh+ then you have this protein, if Rh- then you don't. The gene is dominant, so it only takes one copy to make you Rh+. In this scenario both parents are Rh+, but we don't know if each has one copy of the Rh gene or two copies.Either way, the son got a copy of the Rh+ gene from one of them, making him Rh+ also.
I hope this answers your question. I have attached a couple of links; one is a tutorial about blood groups and types, and the second has a link (left side of the page) for a blood group calculator that will so the problems out for you.
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