If polydactyl is a dominant gene, how is it possible for offspring to have it but not the parents?

Question

If polydactyl is a dominant gene, how is it possible for offspring to have it but not the parents?

In: 10

Answer 1

Most traits aren’t caused by a single gene.

If Gene A and Gene B do nothing on their own, but cause a big blue birthmark if you have them both, then the dad could have Gene A but not Gene B (no birthmark), the moment could have Gene B but not Gene A (no birthmark), and the kid could inherit both (birthmark).

Then there are 16 possibilities with such a double gene instead of 4. Instead of

(A, A); (A, !A); (!A, A); (!A, !A)

you can have

(A, A; B, B); (A, A; B, !B); etc.; etc.

And the real world only gets more complicated from there. Genetics is almost never simple Punnet squares.