Sex between Neandertals and anatomically modern humans tended to follow a specific pattern

"Most Homo sapiens today have a little Neandertal DNA sprinkled through their genome. These genomic signals are the telltale signs that overlapping populations of ancient anatomically modern humans and Neandertals had sex. Exactly what these interactions looked like is a mystery, but a new study suggests that when our species and Neandertals did interbreed, it was primarily between male Neandertals and female humans."

"Neandertal DNA tends to be absent on modern humans' X chromosome. There were other theories as to why that might be, including the possibility that there was some evolutionary disadvantage to the Neandertal X chromosome in humans. Our study allowed us to distinguish between these possibilities."

"If mixing human and Neandertal DNA was disadvantageous, the scientists theorized that Neandertal genomes would show similarly large gaps devoid of ancient human DNA as modern humans do. But that wasn't what we found. Instead, there was much more human ancestry present on Neandertal X chromosomes than the researchers had expected, including in regions that had nothing to do with fitness."

Modern humans carry Neandertal DNA in their genomes, indicating ancient interbreeding between the two species. Researchers investigated why Neandertal DNA is notably absent on the X chromosome in modern humans. By comparing ancient Neandertal DNA with contemporary African populations having minimal Neandertal ancestry, scientists tested whether this absence resulted from evolutionary disadvantage or mating patterns. They found substantial human ancestry on Neandertal X chromosomes, including in regions unrelated to fitness. This discovery indicates the absence of Neandertal DNA on modern human X chromosomes stems from a strong sex bias in ancient mating, specifically between male Neandertals and female humans.

Read at www.scientificamerican.com