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

Everyone has two versions of every gene. Sometimes those versions are identical, sometimes they are not.

If someone has two + versions of the rhD gene, then they will be rhD+.
If someone has two – versions, they will be -.
If someone has a + version and a – version, then they will be +.

Your parents both have a + and a -. They are both +.

You got the – from each of them, so you are a -.