III.9 Mutations occur with greater frequency in the genomes of males.
A number of observations of the representatives of various taxa of organisms with separate male and female sexes have shown that the frequency of mutations that occur in the genomes of males is greater than the frequency of mutations in the genomes of females.This conclusion follows, for example, from comparison of the number of newly formed mutations in the retinoblastoma gene in men and women (Kato et al. 1994).In the relevant studies, all seven observed newly formed mutations in this gene were derived from the father.A number of authors have studied this aspect by comparing the number of substitutions in genes present on autosomes, which occur for 50% of the time of existence of the species in the genomes of males and 50% of the time in the genomes of females, and in genes located on the X-chromosomes, which occur two thirds of the time in the genomes of females, or in genes on Y-chromosomes, which occur throughout their existence only in the genomes of males.It was found by comparison of these frequencies that, in higher primates, including humans, the frequency of mutations in the male genome is approximately six times greater than in female genomes (Shimmin, Chang, & Li 1993).This ratio equals only 2:1 in mice and there is even no difference between males and females in the mutation rate in drosophila (Chang et al. 1994; Drost & Lee 1995).Comparison of these numbers with the ratio of the number of cellular divisions that occur during gametogenesis in males and in females brought most authors to the conclusion that these differences in the frequency of mutations are apparently caused to a considerable degree by the far greater number of cell divisions that occur in many taxa during spermiogenesis than in oogenesis (Drost & Lee 1995).However, it must be admitted that not all authors agree that the difference in the frequency of substitution on autosomes and on X-chromosomes is caused by the greater frequency of mutations in the male genome (Mcvean & Hurst 1997).According to some authors, the lower frequency of mutations (to be more precise, substitutions) in the genes on the X-chromosome could be caused by the greater selection pressure on these genes in male genomes.In contrast to autosomes, they are present here in only a single copy and thus even the recessive mutations can be an object of selection in male genomes.However, it must not be overlooked that, in birds, which are known to have homogamete males (ZZ) and heterogamete females (ZW), a greater frequency of substitution has again been observed on the male Z-chromosome than on the female W-chromosome (Lessells 1997). For another (ultimate) explanation of the higher frequency of mutations in males see also XIV.7.1.