Genetics with Elements of Breeding - M.P. Myhun - 2008

CHAPTER VI. Population Genetics

6.4. Gene and Genotype Frequencies in a Population

When studying the genetic Structure and Variability of populations, METABOLISM/2.html">THE CONCEPT OF the Gene pool is of utmost importance; it represents the aggregate of all genes carried by the individuals of a population. For diploid organisms, the gene pool of a population comprising N individuals consists of 2N haploid genomes. The individuals of such a population possess 2N genes for each locus and N pairs of homologous Chromosomes. Exceptions include Sex Chromosomes and sex-linked genes, which in heterogametic individuals may be represented by a single copy only.

The variability of a gene pool is expressed either by gene frequencies or by genotype frequencies.

Allelic gene frequency is The ratio of its total count across all individuals in the population to the total sum of genes present at the given locus in these individuals.

Genotype frequency is its proportion (fraction) within the total population of individuals.

We observe only phenotypes, not genotypes or genes. However, if the correspondence between genotypes and their respective phenotypes is unambiguous, phenotype frequencies can be used to calculate genotype frequencies, and subsequently, allelic gene frequencies.

This can be illustrated by the distribution of Blood Groups in the MN blood group system. There are three blood groups—M, N, and MN—which are determined by two alleles at a single locus: L^ML^N. According to a survey of 730 Australian Aborigines, blood group M was found in 22 individuals, blood group MN in 216 individuals, and blood group N in 492 individuals.

The genotypes of these people are L^ML^M, L^ML^N, and L^NL^N, respectively.

The frequency of each genotype is determined by the ratio of the number of people with the corresponding blood group to the total number of individuals surveyed. Thus, the frequency of the L^ML^M genotype (blood group M) is 22/730 = 0.030, the frequency of L^ML^N (blood group MN) is 216/730 = 0.296, and the proportion of the L^NL^N genotype is 492/730 = 0.674. If the sample is sufficiently representative, these proportions can be considered characteristic of the gene pool variability of all Australian Aborigines. However, the frequency of the same genotype and its corresponding allelic genes varies significantly across different populations.

Allele frequencies can also be determined from the number of representatives of various genotypes. In the first approach, one must count the number of alleles of the studied type across all individuals in the sample and divide it by the total number of all alleles at the given locus. In the aforementioned example of the human population of Australia with genotypes L^ML^M, L^ML^N, and L^NL^N, the total number of allelic genes at the studied locus is 2N = 730 * 2 = 1460. Individuals with the L^ML^M genotype carry two L^M alleles, those with L^ML^N carry one L^M and one L^N allele, and those with LⁿLⁿ carry two L^N alleles. The number of L^M alleles in the given Aboriginal sample is (22 * 2) + 216 = 260, and the frequency of the L^M allele is 260 / 1460 = 0.178. Similarly, for the L^N allele: (492 * 2) + 216 = 1200; 1200 / 1460 = 0.822.

Considering that homozygotes carry two identical alleles and heterozygotes carry one allele of each type, allele frequencies can also be calculated directly from genotype frequencies. In this case, the frequency of the studied allele is equal to the frequency of the corresponding homozygotes plus half the frequency of the heterozygotes. In our example, the frequency of the L^M allele calculated by this method is: 0.030 + 0.296 / 2 = 0.178, and the frequency of the L^N allele is 0.674 + 0.296 / 2 = 0.822.

It should be noted that gene pool variability is more conveniently measured by allele frequencies rather than genotype frequencies, because the number of distinct alleles is always smaller than the number of genotypes. If There are two alleles at a given locus, the number of possible genotypes is three; if there are three alleles, the number of genotypes is six. For n alleles at a single locus, the number of possible genotypes is calculated by the formula: n(n+1)/2