text: yes, the blood type is based on the genes of the parents, so there are 2 parts. being positive is a dominant trait and being negative is recessive.  so the you also have to look at the grandparents.  if one of them had both recessive genes and the other had postive, that would mean that one of the parents could have passed on the recessive gene and the other a dominant making that person be positive, but still able to pass on the recessive gene.