The first place of reduced Neanderthal ancestry was on chromosome X, one of the two sex chromosomes. This reminded me of a pattern that Nick Patterson and I had run into in our work on the separation of human and chimpanzee ancestors in a study we had carried out together and published years before.33 There are only three copies of chromosome X in any population for every four other chromosomes (because females carry two copies and males only one, in contrast to two copies in each sex for most of the rest of the chromosomes).
