How can a person of blood type B+ have an O- child?

Heterzygous B- mates w/ a heterzygous B- or a homozygous O-

Simple as that.

Rh+ means the blood reacts to the Rhesus serum test. Rh- is dominant. This chart http://mistupid.com/health/bloodinherit.htm shows the possible results.

And this chart shows the possibilities, depend on other parents type and O is scattered all through the chart. The only way to make it impossible is for other parent to be O or A.

An on the chance this chart has errors, you might look at others.http://www.google.com/search?q=blood+type+inherita...

this has got to do with alleles. B+ can be heterogeneous or a homogeneous.

heterozygous B -------> B alleles only

homozygous B ---------> B and O alleles

DAD: BB MUM Bb

CHILD: BB BB BB Bb ( all have blood B)

but if DAD BO MUM BO

CHILD: BB BO BO OO ( the OO will have an O blood, only 25% chance of that happening )