Molecular Anthropology - X-linked Studies

X-linked Studies

The X-chromosome is also a form of nuclear DNA. Since it is found as 1 copy in males and 2 non-identical chromosomes in females it has a ploidy of 1.5. However, in humans the effective ploidy is somewhat higher, ~1.7, as females in the breeding population have tended to outnumber males by 2:1 during a large portion of human prehistory. Like mtDNA, X-linked DNA tends to over emphasize female population history much more than male. There have been several studies of loci on X chromosome, in total 20 sites have been examined. These include PDHA1, PDHA1, Xq21.3, Xq13.3, Zfx, Fix, Il2rg, Plp, Gk, Ids, Alas2, Rrm2p4, AmeIX, Tnfsf5, Licam, and Msn. The time to most recent common ancestor (TMRCA) ranges from fixed to ~1.8 million years, with a median around 700ky. These studies roughly plot to the expected fixation distribution of alleles, given linkage disequilibrium between adjacent sites. For some alleles the point of origin is elusive, for others, the point of origin points toward Sub-Saharan Africa. There are some distinctions within SSA that suggest a smaller region, but there is not adequate enough sample size and coverage to define a place of most recent common ancestor. The TMRCA is consistent with and extends the bottleneck implied by mtDNA, confidently to about 500,000 years.