Mullers ratchet

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Muller's ratchet A model, proposed by H. J.Muller in 1964, which explains the advantage enjoyed by sexually reproducing organisms over those reproducing parthenogenetically in terms of the ability to eliminate deleterious mutations. The best-adapted genotypes are occasionally lost from a population. If the members of that population reproduce sexually, the genotype can be recreated by crossing-over, but if reproduction is parthenogenetic it can be recreated only by favourable mutation. Favourable mutations being less common than deleterious mutations, each loss of a best-adapted genotype acts like a ratchet to increase the load of deleterious mutations until it becomes intolerable and the clone dies. The ratchet prevents any line in an asexual population from reducing its deleterious mutation load to a level lower than that of the line with the lowest load.