Variance of protein heterozygosity in different species of mammals with respect to the number of loci studied.

Analysis of published data on protein heterozygosity of 321 species of mammals shows that it varies from 0 up to 22%, an average species being heterozygous at 5% of its protein-coding loci. Many attempts have been made to explain the observed differences in protein heterozygosity, relating its value to various species-, population-, or environment-specific parameters. In this work it is shown that the wide scatter of protein heterozygosity in different species of mammals can be explained by the small numbers of loci studied (usually 20-30). It is shown that with an increasing number of studied loci, the mean of the heterozygosity does not change, while its variance among different species decreases in accordance with a Poisson distribution. The true heterozygosity of the whole protein-coding region of the mammalian genome is thus characterized by a narrow spread around the mean. This means that the true heterozygosity of the protein-coding region is similar in all mammalian species. Its value can be viewed as the threshold level of variability of the protein-coding region of mammals, which characterizes the permissible level of erosion of genetic information of species and is maintained by stabilizing selection in natural ecological niches.