Neanderthals seemed to have a thing for modern human women

"Not every modern human has the same set of Neanderthal DNA, however; different people will, by chance, have inherited different fragments. But there are also some areas, termed "Neanderthal deserts," where none of the Neanderthal DNA seems to have persisted. Notably, the largest Neanderthal desert is the entire X chromosome, raising questions about whether this reflects the evolutionary fitness of genes there or mating preferences."

"It turns out there's also a strong bias toward modern human sequences there, as well, and the authors interpret that as selective mating, with Neanderthal males showing a strong preference for modern human females and their descendants."

"Given how long modern humans and Neanderthals had been evolving as separate populations, some degree of genetic incompatibility is definitely possible. Lots of proteins interact in various ways, and the genes behind these interaction networks will evolve together-a change in one gene will often lead to compensatory changes in other genes in the network."

Modern humans and Neanderthals interbred as humans expanded from Africa, leaving Neanderthal DNA throughout modern genomes. However, distribution is uneven, with "Neanderthal deserts" where no Neanderthal DNA persists, most notably the entire X chromosome. Researchers at the University of Pennsylvania analyzed this pattern and found a reciprocal bias: modern human X chromosome sequences are overrepresented in Neanderthal genomes. This asymmetry suggests selective mating rather than random inheritance. Genetic incompatibility likely explains the pattern, as populations evolving separately accumulate divergent changes in interacting genes. When incompatible genes from one population enter another, they disrupt genetic networks and reduce fitness, causing them to be selected against and lost over generations.

Read at Ars Technica