Blood Type

by Stephen Greenleaf

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Student Question

Can a child have B+ blood type if both parents have O blood types?

Expert Answers

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There are two parts to how we commonly type blood: the part that deals with antigens (the part of blood type that is A, B, AB, or O) and Rh factor (the positive or negative after the letters). The antigen portion of the blood type says what antigens the blood cells have or don't have. A has A antigens, B has B antigens, AB has both A and B antigens, and O has neither antigen. For the Rh factor, it is either present (positive) or not (negative). A parent can only pass down an antigen and Rh factor they possess. Parents with AB blood can pass down either the A or the B marker.

In this specific case, neither parent has either antigen, so a child would have to have an O blood type. For the child to have a B blood type, one parent would have to have either AB or B as their blood type. The Rh factor is similarly passed down. For the child to be Rh positive, there would have to be an Rh positive parent. It would be impossible for an O- mother and an O+ father to have a child with B+ blood. The only possible blood types for these parents would be O- (only possible if the father had an Rh negative gene) or O+.

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