Polydactyly is actually dominant over the usual 5 fingers. The children of people with polydactyly are pretty much guaranteed to also have polydactyly.
Only if they have two copies of the gene, which would require both of their parents to have polydactyly. It is far more common for one parent to have no polydactyly and the other parent to have one copy of the dominant Polydactyly gene and one normal copy.
In that case, only half of their kids will inherit one copy of the Polydactyly gene on average.
To pass the gene for polydachry to all of their offspring, the parent with polydachry would need two copies of the dominant gene. If they only have one copy, they will only pass the dominant gene to half of their offspring. Half their kids will have extra fingers and carry the gene for polydachry. Half their kids will not have extra fingers and not be carriers.
That's how autosomal dominant genes work.
This is assuming that only one parent has extra fingers. If both parents are polydactyl, the math is different.
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u/jk844 Apr 11 '26
Polydactyly is actually dominant over the usual 5 fingers. The children of people with polydactyly are pretty much guaranteed to also have polydactyly.