Gene Relationships in Two Species of Mice with Reference to Their Possible Evolutionary Significance

The results obtained in crossing Mus bactrianus with strains of Mus musculus carrying a number of recessive gene mutations indicate that, in general, the wild type complex is similar in the two species. The basic genes investigated are, for the most part, probably the same, but with their expression influenced by different modifiers. With certain dominant musculus gene mutations the situation differs somewhat. Although dominant over the wild type bactrianus, the expression in the hybrids is much lessened and in the direction of the normal condition found in both forms. The wild type complex found in a species is probably the balanced result of an evolutionary process and, since recessives seem most frequently to represent inactivations of genes, for physiological reasons is similarly dominant over the recessive genes of another species. On the other hand, the expression of a dominant gene, which must bear such a relation to the wild type complex from which it originated that its expression is permitted away from normality, is as a result often reduced toward the normal condition when introduced into a second species with a somewhat different complex. The balance in mice was probably attained very early, and in consequence the ancestral species was essentially stable before divergence into the closely related forms Mus bactrianus and Mus musculus.