system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If polydactyl is a dominant gene, how is it possible for offspring to have it but not the parents?
gpt
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.)