GENETIC LOAD DUE TO MUTATIONS WITH VERY SMALL EFFECTS

Muller (1950) argued in his famous paper "Our Load of Mutations" that the borderline between deleterious and neutral mutations is vague and that it must be a "cline" rather than a line. To stay on the "ultraconservative side", he disregarded genes with a homozygous deleterious effect below about ten percent. On the other hand, recent investigations on molecular evolution (Kimura 1968, 1969b; King and Jukes 1969; Crow 1969) suggest that numerous neutral and nearly neutral mutations are constantly arising in the population. Also, it has been shown by Mukai (1964, 1969) that the occurrence of mutations with fairly small deleterious effects is much more common than previously thought. These facts indicate that the mutation rate for genes with very small effects (having selection coefficients of the order of the reciprocal of the effective population number) must also be high. For such a class of mutants, conventional treatments of genetic loads based on deterministic models can not be applied, since such mutants are subject to random genetic drift. A theoretical study of mutational load in small populations has been made by Kimura et al. (1963) using the classical model of 2 alleles per locus but taking account of the effect of random sampling of gametes. Their results show that mutants with very small deleterious effects sometimes create more load than those with larger effects by spreading through the population due to random frequency drift. The purpose of the present paper is to pursue further the problem of genetic load due to mutations with very small effects using a more realistic model of molecular mutation. Namely, we use the model of "infinite sites" (Kimura 1971) with steady flux of irreversible mutations.

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