Can your determine paternity just by matching blood types?

Question

For instance: If both have O blood types.

Answer 1

Generally no. There are a few reliable rules (although even these have rare exceptions and complications) especially if you know one parent's blood type and the baby's for certain, which could _rule out_ some individuals.

Notably:

if one parent is O and the child is A or B, the other parent can't also be O. More generally, an A child must have at least one parent who is A or AB, and a B child must have at least one parent who is B or AB

if either parent is O, the child can't usually be AB, likewise if either parent is AB, the child can't usually be O

if one parent is a negative blood group and the child is positive, the other parent can't also be negative (the reverse rule doesn't work)

As I said, there are occasional exceptions to almost any simple rule of inheritance. For example, it was recently found that in some cases a different from usual genetic basis for AB type could be inherited from one parent alone, meaning that a handful of AB types have been born to AB parents despite the other parent being O type.

Answer 2

Blood type cannot determine the father.  Only DNA.

Answer 3

no the blood type tells you nothing. you have to go by dna sorry

Answer 4

Perhaps if one of the guys had a rare blood type and the other didn't... and the baby also had the rare blood type.  But even then, if the other guys carries the rare bllod type as a recessive gene... I guess it wouldn't work.  

O blood type is very common.  Sounds like DNA is the only way to go.