We don’t see people today with Neanderthal Y chromosomes or mtDNA. I keep hearing people argue that this means that mating between Neanderthal males and AMH females must have produced sterile males, or that matings between AMH men and Neanderthal women were all sterile, or whatever.
That is not necessarily the case. A slight disadvantage is all that would be required to totally eliminate Neanderthal Y-chromosomes or mtDNA.
Imagine that a Neanderthal Y-chromosome reduces the bearer’s fitness by 1%, and that the original frequency of Neanderthal Y chromosomes (after admixture) was 2%.
It’s been something like 1500 generations. The expected frequency is 5.67 x 10-9. In real life it would probably have fluctuated to zero, and of course stayed there.
Understand and remember.