GAW: The Tzitz Eliezer has a T’shuvah (in Chelek 14, I think) about using blood type for paternity. He knew the medicine, but rejected it based on the Aggadta that a man gives the white part of the baby (bones and skin) while a woman gives the red (blood), thus blood type can’t prove paternity.
It’s fairly obvious and proven that when the Gemara says that the “red portion” of the blood comes from the mother that it cannot mean blood type. Were that to be the case, then you could never have a case of maternal (or full) siblings with different blood types.
While all of my kids have my wife’s blood type (and not mine), I also know that my sister and I (and we’re full siblings) have different blood types.
R’ Moshe Shternbuch has a T’shuvah (chelek 4 in the middle, I think) that says that blood type and DNA can’t prove paternity because maybe the doctors will realize 50 years from now that this really doesn’t work.
WADR to R. Shternbuch, the possibility of this is highly unlikely, since it’s been proven over and over again.
The Wolf