Interdeme Selection Controlling a Polymorphism in the House Mouse

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Abstract
Certain populations of house mice carry a gene, t, which makes homozygous males sterile. There are maintained in the population because heterozygous males produce sperm, a large predonderance of which carry the mutant allele. An investigation has been made of the fate of such genes in natural populations. The results are (1) in small families the mutant allele may be lost by random fixation of wild type alleles. (2) More often it is the t allele which becomes fixed in males. This results in extinction of the family. (3) The rate of extinction compared with the rate of fixation of wild type genes increases as family size increases. (4) The polymorphism will be stable in large families and unstable in small ones but such small families run a great risk of extinction. (5) The most favorable condition for maintenance of sterile t alleles in small populations is that of an intermediate transmission ratio as compared with the high ratios expected in lethal alleles. This comparison agrees with the observed segregation ratios in lethal and sterile alleles from wild populations. The sterile t alleles are anexample of the control of the genetic composition of a species by interdeme selection operating through the frequent extinction of demes carrying such alleles.