Abstract
The spread of a "cream" mutant in a wild population of house mice is reported. The hypothesis that the gene responsible for the colour, extreme chinchilla, ce, has spread because of linkage with a major gene for warfarin-resistance, is tested by a linkage backcross. The results prove that a major gene does exist, that it is very closely linked with frizzy, fr, in chromosome 7, which in turn is linked with ce, that it is fully dominant in females at 4 months of age, and that its partial dominance in males is under the control of modifiers. The symbol War is proposed for the gene. Its position in chromosome 7 is analagous with the position of the resistant gene, Rw2, in the rat in the analagous chromosome. The adaptive significance of the finding is discussed, as also are reports of certain other mutants in wild populations of mice.