Question

How does the difference in allelic frequencies between two starting populations affect the changes in frequencies over time?

Answer 1

Allele frequency

• Allele frequency is the concept used to quantify genetic variation.
• It is defined as a measure of the commonness of a given allele in a population, that is, the proportion of all alleles of that gene in the population that are specifically this type

An allele is an alternative form of a gene. If a gene corresponds to a specific sequence of nucleotides along a DNA molecule, the alleles represent the different sequences of nucleotides that are possible for that particular locus.

Often the term ‘gene’ is used synonymously with ‘allele’ and, hence, ‘gene frequency’ is sometimes used synonymously with ‘allele frequency’. Allelic differences at a single locus in a population indicate genetic variation. This genetic variation needs to be quantified for different genes and for different individuals or populations.

Calculating the allele frequency

P(A) = [2(AA) + (Aa)]/2n

• Twice the number of homozygous genotypes with that allele (because homozygotes carry two copies each of the same allele),
• plus the number of heterozygous genotypes with that allele (because heterozygotes carry only one copy of a particular allele),
• divided by two times the total number of individuals in the sample (because each individual carries two alleles per locus)

Note that any result obtained with this formula will only be an estimate of the total allele frequency in the population, because only a sample of individuals is usually studied. However, if the sampling of individuals is well done, that is, the size of the sample is sufficiently large, then it can be assumed that our calculation is close to the true allele frequency. As a rule of thumb, allele frequency estimates should be performed, where possible, on samples of 100 individuals or more.

Population genetics describes the genetic composition of a population, including allele frequencies, and how allele frequencies are expected to change over time. The Hardy-Weinberg law describes the expected equilibrium genotype frequencies in a diploid population after random mating. Random mating alone does not change allele frequencies, and the Hardy-Weinberg equilibrium assumes an infinite population size and a selectively neutral locus.

In natural populations natural selection (adaptation mechanism), gene flow, and mutation combine to change allele frequencies across generations. Genetic drift causes changes in allele frequency from random sampling due to offspring number variance in a finite population size, with small populations experiencing larger per generation fluctuations in frequency than large populations. There is also a theory that second adaptation mechanism exists – niche construction. According to extended evolutionary synthesis adaptation occur due to natural selection, environmental induction, non-genetic inheritance, learning and cultural transmission. An allele at a particular locus may also confer some fitness effect for an individual carrying that allele, on which natural selection acts. Beneficial alleles tend to increase in frequency, while deleterious alleles tend to decrease in frequency. Even when an allele is selectively neutral, selection acting on nearby genes may also change its allele frequency through hitchhiking or background selection.

While heterozygosity at a given locus decreases over time as alleles become fixed or lost in the population, variation is maintained in the population through new mutations and gene flow due to migration between populations.

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