PMC:3480682 / 5084-7853 JSONTXT

{"project":"2_test","denotations":[{"id":"23105934-17248636-44845812","span":{"begin":793,"end":795},"obj":"17248636"},{"id":"23105934-12689793-44845813","span":{"begin":2350,"end":2352},"obj":"12689793"},{"id":"23105934-12689793-44845814","span":{"begin":2765,"end":2767},"obj":"12689793"}],"text":"F-statistics\nWright [12] introduced F-statistics (FST, FIT, and FIS) as a tool for describing the partitioning of genetic diversity within and among populations that are directly related to the rates of evolutionary processes, such as migration, mutation, and drift. Specifically, F-statistics can be defined in many different ways: in terms of variances of allele frequencies, correlations between random gametes, and probabilities that two gametes chosen have different alleles. Depending on the relativity to the subpopulation or to the total population, FST, FIT, and FIS are defined, where subscript IS refers to 'individuals within subpopulations,' ST to 'subpopulations within the total population,' and IT to 'individuals within the total population.'\nFollowing the work of Cockerham [16], F-statistics are defined in terms of the variance components - that is, the total variation in the genetic data is broken down into three components: (a) between subpopulations within the total population (we sometimes say 'between populations'); (b) between individuals within subpopulations; and (c) between gametes within individuals. FST, FIT, and FIS are defined as the expectations under the model of a/(a + b + c), (a + b)/(a + b + c), and b/(b + c) and estimated by the corresponding sample values [17, 18]. Here, it is perhaps pertinent to mention that when Weir and Cockerham [18] presented these definitions, they assumed a model consisting of an ancestral population from which subpopulations have descended in isolation under the same evolutionary processes. Thus, it is meaningful to have a single measure of population structure; that is, a global FST, which is an average over subpopulations. However, in identifying candidate loci under natural selection, evidence for locus-specific selection is of interest, and thus the estimators of locus-specific FST will be described in the next section.\nOften, readers may be confused with several terms appearing in population genetics. Wright [12] interprets FST as a measure of the progress of the subpopulation towards the fixation of one allele of each locus in the absence of mutation and hence called a 'fixation index.' FST is also interpreted as a measure of shared ancestry with the subpopulations, relative to that in the population, and is thus called the 'coancestry coefficient' [19]. Therefore, if the value of FST is small, it means that the allele frequencies within each subpopulation are similar; if it is large, it means that the allele frequencies of subpopulations are different. On the other hand, FIS or FIT is defined as the correlation between two gametes that form a zygote relative to the subpopulation or population, and thus, FIS (or FIT) is called the 'inbreeding coefficient' [19]."}