Ylinked Genes

Because the Y chromosome is transmitted unisexually (through males), it provides an excellent example of how a gender-based asymmetry in genetic transmission can lead to contention among genes and to selection pressures for conflict resolution. Suppose you were a self-interested gene located on the Y chromosome. Your prospects for representation in the next generation would be enhanced if somehow you could promote the production and dissemination of your own Y-carrying gametes at the expense of other gametic types. This "Y-linked drive" would be to your short-term advantage, but also would bring you into immediate conflict with the preferred strategies of genes housed on the X chromosome (one third of which in any population are passed through males). Your strategy also would be received poorly by nondriving Y chromosomes, by mitochondrial genes whose inheritance is matrilineal, and by X-linked and autosomal genes that have a vested interest in seeing the continued production of both male and female genders through which they are transmitted.10 Indeed, an extreme form of Y-linked drive ultimately could prove suicidal because continued production of nothing but males inevitably would result in species' extinction.11

Nonetheless, captivated by the prospects of short-term transmission success, and blinded to the ramifications of your actions, you might choose to forge ahead with a strategy of Y-linked drive. If you didn't, some other ultra-selfish Y-chromosome alleles in the population surely would. Given the short-term selective advantage of Y-linked drivers over their nondriving counterparts, why aren't they observed more often in humans and other species? Perhaps Y-driving alleles are observed infrequently in extant species because the populations in which they arose have been meiotically driven to extinction; perhaps they are mutationally rare (see note 8); or, perhaps countervening selection pressures on other chromosomal genes have thwarted their spread.

William Hamilton raised a hypothesis that combines elements of these latter two explanations, and also might account for the long-standing enigma of why the Y chromosome alone carries so few functional genes. According to Hamilton's theory, the relative genetic inertness of the Y chromosome is due to the suppressive effects of autosomal (or other non-Y) modifier genes that have evolved under the influence of natural selection to stymie Y-linked meiotic drive behavior. Thus, evolutionarily, the Y chromosome has been stripped of most operationality beyond that minimally required to initiate the developmental cascade of maleness. Although this hypothesis remains unproved, it exemplifies how, in principle, natural selection might resolve internecine genomic conflicts and thereby curb the strategic maneuverings of any unduly self-interested genetic drivers on the Y.12

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