Quote from ms.pebbles
Do children always have to have the blood of their father??? If you're blood is not the same with your father, does this mean you're not his child????
You've been watching too many soap operas
Because only in soaps is blood type the guaranteed indicator of paternity (or lack thereof).
Remember that the child gets one gene from each parent, and that the O gene is recessive so you'd need two of them to be a type O. So someone with AO or AA is type A. If the parents are both AO (and they may know they're A, but not AO), the kid can be O and still be their kid because he got an O from each parent.
Blood type is very unreliable as a means of confirming or denying paternity. True, there are a few cases where it could rule out a father...for example, if the parents are both Type O but the baby is Type B, then someone's got some explaining to do
But there are so many other combinations possible where the father's type doesn't have to match the kid's though they are definitely parent-child. Another example: if dad is AB and mom is BO, then the kid can be AO (A), BO or BB (B) or AB (AB).
Last, blood types can not confirm paternity. The kid's blood type may match the father's (or be a possible combination of the parents'), but that doesn't mean that he really is the father.
It's all basic ABO info and is in your med-surg text. Rh factor works along the same guidelines.
Hope this helps!