Two-fold genetic cost of sex
In sexual reproduction, the female basically dilutes to one half the genes transferred through progeny to future generations. This phenomenon, denoted as the two-fold genetic cost of sex (cost of meiosis) (Williams 1975; Uyenoyama 1984), is especially apparent when compared with the fitness of individuals reproducing through self-fertilization and individuals reproducing in the usual biparental manner (Fig. XIII.2). In this case, the organism provides both its chromosome sets to each of its progeny. This means that the gametes that its progeny will produce will contain only genes coming from itself. In contrast, a sexually reproducing individual passes only half of its genes (one chromosome set) on to its progeny. As a consequence, half of the gametes of the progeny will contain only a copy of any gene derived from the other parent. If a sexually reproducing individual were to transfer to the gene pool of the next generation the same number of copies of its genes as an individual reproducing by self-fertilization, then it would have to produce exactly twice as many progeny.
The two-fold genetic cost of sex (and the two-fold ecological cost of sex) is fully applicable only to organisms with certain, quite specifically defined reproductive systems (Uyenoyama 1984). Amongst other things, it is necessary to take into account that, from the standpoint of the gene (or allele) it, to a substantial degree, matters little that the sexually reproducing female passes only one copy of its genes on to the next generation. The gene for sexual reproduction is understood here to mean an allele of any gene that, compared to any other allele of the same gene, increases the probability that its carrier will reproduce sexually. In sexual reproduction, the male brings a copy of its own alleles of all the genes to the zygote; however, for the gene for sexual reproduction it will have to consist of a copy of the same allele (gene for sexual reproduction) as the female has in its genome, otherwise the male would not reproduce sexually. Thus, from the standpoint of the gene for sexual reproduction, the two-fold cost of sex does not exist – the young of parthenogenetic and sexually reproducing females receive the same numbers of its copy.