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

Mom has O+ and O-. O+ wins

Dad has O+ and O-. O+ wins

Their offspring could get

1. O+ and O+, O+ wins
2. O+ and O-, O+ wins
3. O- and O-, O- wins

You are possibility #3

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Edit: This is a vast oversimplification. Without getting too much into genetics, statistics, dominant trait etc.