From hybrids between other mammal species, we know that many genes that affect fertility tend to be found on the X chromosome, and this usually affects males. It has been suggested that human/Neanderthal offspring, especially males, may have experienced fertility problems, which could also explain why we see less Neanderthal ancestry among genes that are mainly switched on in the testis. If this is true, our Neanderthal ancestry traces back mostly through hybrid women, not men.
