Both of my parents have rhD positive blood. I came out with rhD negative blood. How?

Question

Mum: O+
Dad: B+
Me: O-

In: 673

Answer 1

Your parents both have a dominant copy of the rh+ gene and a recessive copy of the rh- gene.

If one of your parents passed on a positive gene, your blood would be rh positive. That wasn’t the case. Both of your parents passed on the rh negative gene, which makes you rh negative. There’s a 1/4 chance of that happening. If your parents had four kids, statistically, one should have only two rh positive genes, two should have positive and negative (which will express positively) and one (you) would have two negative genes.

Tldr: your parents each have an rh negative copy of genes in addition to their rh positive copy. They passed on the negative instead of the positive.