Re: Can two O+ parents have a B- child?

The quick and simple answer is no, two O+ parents cannot have a B- child, no matter what the genotypes of the child's grandparents were.

There are two independent blood systems mentioned in your question; the ABO blood types, and the rhesus system. (the + and -) The ABO blood types are co-dominant antigens expressed from three major alleles (of which, everyone has two.) The O allele is recessive to both the A and B allele, so if you have the O phenotype, you don't have either the A or B allele in your genome. So, two O parents (both of which don't have the B allele in their bodies,) cannot have a child that suddenly does have the B allele. This would prove that the O father in your example is definately not the parent of this child.

The rhesus system is more complicated than the ABO antigen system. If you're interested, here's a web site that explains a little bit about it. http://scarf.uth.tmc.edu/public/groups/004.html

Depending on the genotypes of the parents, they could have a child that was Rh-, but in this case, the B blood type precludes the O+ male from being the father.

If you're still interested, here's another webpage that explains things in more detail. http://www.umds.ac.uk/tissue/bludgrp2.html#Abbs

Hope this has helped.