Time in the womb and the environmental factors associated are indeed possible contributors.
But it would not be an identifiable mutation except under extreme cicumstances. Most likely modify the resulting proteins produced by the existing alleles.
Would not discount it. But again, it’s not really a transmitted genetic trait; it’s something acquired, and possibly treated.
Mammals evolved needing two to tango. And it can’t be two with the same equipment set. Or predilection to the same equipment. Maybe could work some other way; there many examples in the lower phyla. But not ours.
Not sure I follow that, but then I am not a scientist.
I would argue the results of such an atypical prenatal development that we are discussing would not be difficult to identify, and are often readily apparent even to a lay person. I.e., there is reason to believe there was indeed a mutation.
With that, rather than my earlier reference to a “mistake of nature”, it would have been more precise to say nature reliably produces those results and our effort should be to identify the mechanisms of causation.