Blood Types, Inheritance and Paternity

Long before complex DNA testing, blood tests and blood types were able to help determine paternity. That is because the main blood type of a child is directly determined by the blood types of the parents.

The four main blood types are A, B, AB and O. Each parent has two "letters" in their blood type and gives one to their child. The combinations are:

AA = blood type A
AO = blood type A
BB = blood type B
BO = blood type BO
AB = blood type AB
OO = blood type O

So if a mother was AA and a father was BB, the child could only be an AB. If the mother was OO, and the father was OO, then the child could only be OO. If the mother was AA, and the father was AB, the child would have a 50/50 chance of either being AA or AB.

So if for example the mother was AA, and the child was AA, and the "father" was BB, this would be proof that the father was not the biological father.

The Rh factor for blood works in a similar way. Each person has 2 Rh "settings" in their blood, either positive or negative. The only way to be Rh negative is to have BOTH of those be negative. So the combinations are:

++ = Rh positive
+- = Rh positive
-- = Rh negative

So if both parents are ++, then the child MUST be ++. If both parents are --, the child MUST be --. If one parent is ++ and the second parent is +-, the child has a 50/50 chance of being either ++ or +-. But a child who is -- cannot come from a parent who is ++, because the child must inherit at least one of those +s.

So to take as an example:
Father: Rh positive
Mother: Rh negative

We know the mother MUST be --. That's the only way to be negative. However, the father COULD be +-. That is one type of Rh positive. So the kids could randomly be Rh positive -+ (negative from mom, positive from dad) OR they could be Rh negative -- (negative from mom, negative from dad). It's random chance which of the two halves of dad's set they will get.

More about Blood Types
Blood Types and Transfusions
Blood Type Distributions Around the World
Blood Types and Illnesses
Important Types of Blood
The Basics of Blood
Evolution of Blood Types

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