| Method |
Researchers measured the difference in the percent matching
by a statistic D(H1, H2, Neandertal, chimpanzee)
(SOM Text 15) that does not differ significantly
from zero when the derived alleles in the Neandertal
match alleles in the two humans equally
often. If D is positive, Neandertal alleles match
alleles in the second human (H2) more often,
while if D is negative, Neandertal alleles match
alleles in the first human (H1)more often. They performed
this test using eight present-day humans:
two European Americans (CEU), two East Asians
(ASN), and four West Africans (YRI), for whom
sequences have been generated with Sanger
technology, with reads of ~750 bp that they mapped
along with the Neandertal reads to the chimpanzee
genome. To obtain an independent estimate of the proportion of
Neandertal ancestry of non-Africans (f), researchers fit
a population genetic model to the D statistics in
Table 4 and SOM Text 15 as well as to other
summary statistics of the data. Assuming that
gene flow from Neandertals occurred between
50,000 and 80,000 years ago, this method
estimates f to be between 1 and 4%, consistent
with the above estimate (SOM Text 19) |