Genetic ancestry, population admixture, and the genetic epidemiology of complex disease.

Race/ethnicity is a social construct incorporating biological, sociocultural, psychological, and behavioral components, and there is no genetic definition of race.1 In contrast, genetic ancestry is a genetics concept that describes the architecture of genome variation between populations.2 All genetic variation begins locally, as a new mutation in an individual, and thus, all new mutations are initially geographically localized. By virtue of this shared population history, there is a correlation between genetic variants found in one geographic region as compared with those found elsewhere. It follows that variation that is common across human populations separated by geography tends to be old, and many of these variants were likely present at the time of human expansion out of Africa 100 000 years ago.2 In contrast, more recent mutations have not had the time to spread between populations and tend to be of lower frequency, unless under unusual selection pressure and localized to geographic populations. When the recombination history has been short ( 20 generations or so), these recent variants will tend to remain in linkage disequilibrium with other markers located on ancestral haplotype on which the new mutation occurs.