If polydactyl is a dominant gene, how is it possible for offspring to have it but not the parents?
In: 10
A new mutation arose in the person’s mother’s eggs or in the person’s father’s sperm. That mutation changed the gene responsible for polydactyly, but only in that one egg or sperm cell, and not in the mother or father’s whole body. When the person was conceived, that gene became part of their genetic code, and that code was used as a guide for how to build their body – in this case, with extra fingers.
(I am taking your premise as given. Not all polydactyly *is* caused by a single dominant gene.)
Early mechanical intervention. Parents could have had it mechanically removed early on in childhood preventing it’s expression. It may require environmental triggers or multiple genes. It may be a novel mutation.
I can’t say if this applies to polydactyly but reduced “penetrance” of a gene can lead to having a dominant allele that is not expressed.
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.
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