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Coalescent theory is a model of how alleles sampled from a population may have originated from a common ancestor.In the simplest case, coalescent theory assumes no recombination, no natural selection, and no gene flow or population structure, meaning that each variant is equally likely to have been passed from one generation to the next.
If dioecious organisms are heterogametic and the gene locus is located on the X chromosome, it can be shown that if the allele frequencies are initially unequal in the two sexes [e.g., XX females and XY males, as in humans], f′(a) in the heterogametic sex 'chases' f(a) in the homogametic sex of the previous generation, until an equilibrium is ...
In the basic multispecies coalescent model, the species phylogeny is assumed to be known. Complete isolation after species divergence, with no migration, hybridization, or introgression is also assumed. We assume no recombination so that all the sites within the locus share the same gene tree (topology and coalescent times).
Selection does not act on a single locus, but on a phenotype that arises through development from a complete genotype. [20] However, many population genetics models of sexual species are "single locus" models, where the fitness of an individual is calculated as the product of the contributions from each of its loci—effectively assuming no ...
More recent models have focused on partial sweeps, and on soft sweeps [3] - i.e., sweeps that start from standing variation or comprise multiple sweeping variants at the same locus. All of these models focus on adaptation through genetic changes at a single locus and they generally assume large changes in allele frequencies.
As a simple example of mutation-selection balance, consider a single locus in a haploid population with two possible alleles: a normal allele A with frequency, and a mutated deleterious allele B with frequency , which has a small relative fitness disadvantage of .
The simplest genetic model involves a single locus with two alleles (b and B) affecting one quantitative phenotype. The number of B alleles can be 0, 1, or 2. For any genotype, ( B i , B j ), where B i and B j are either 0 or 1, the expected phenotype can then be written as the sum of the overall mean, a linear effect, and a dominance deviation ...
Bacillus subtilis is a model organism used to study bacterial chromosome replication. Replication of the single circular chromosome initiates at a single locus, the origin . Replication proceeds bidirectionally and two replication forks progress in clockwise and counterclockwise directions along the chromosome.