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These random fluctuations within the allele frequencies can lead to the fixation or loss of certain alleles within a population. To the right is an image that shows through successive generations; the allele frequencies fluctuate randomly within a population. The smaller the population size, the faster fixation or loss of alleles will occur.
The remaining copy of the tumor suppressor gene can be inactivated by a point mutation or via other mechanisms, resulting in a loss of heterozygosity event, and leaving no tumor suppressor gene to protect the body. Loss of heterozygosity does not imply a homozygous state (which would require the presence of two identical alleles in the cell).
In genetics, a selective sweep is the process through which a new beneficial mutation that increases its frequency and becomes fixed (i.e., reaches a frequency of 1) in the population leads to the reduction or elimination of genetic variation among nucleotide sequences that are near the mutation.
It has been shown that it takes only "one migrant per generation" to prevent populations from diverging due to drift. [1] Populations can diverge due to selection even when they are exchanging alleles, if the selection pressure is strong enough. [2] [3] Gene flow is an important mechanism for transferring genetic diversity among populations.
In these simulations, alleles drift to loss or fixation (frequency of 0.0 or 1.0) only in the smallest population. Assuming genetic drift is the only evolutionary force acting on an allele, after t generations in many replicated populations, starting with allele frequencies of p and q , the variance in allele frequency across those populations is
The main difference between soft and hard selective sweeps lies in the expected number of different haplotypes carrying the beneficial mutation or mutations, and therefore in the expected number of haplotypes that hitchhike to considerable frequency during the selective sweep, and which remain in the population at the time of fixation.
[1] [11] [12] The principles of population genetics, established by J.B.S. Haldane, R.A. Fisher, and Sewall Wright, created a mathematical approach to analyzing gene frequencies that contributed to the development of Kimura's theory. Haldane's dilemma regarding the cost of selection was used as motivation by Kimura.
In general, alleles drift to loss or fixation (frequency of 0.0 or 1.0) significantly faster in smaller populations. Genetic drift is the change in the relative frequency in which a gene variant occurs in a population due to random sampling. That is, the alleles in the offspring in the population are a random sample of those in the parents.