Umbrous: A case of dominance modification in mice

Summary

The gene umbrous is a modifier of the expression of agouti. It has no visible effect in non-agouti animals, but makes agouti mice darker. Heterozygous agouti is affected more than homozygous agouti. Heterozygous umbrous has a definite effect, but it is not so marked as that of homozygous umbrous. In homozygous umbrous mice, homozygous and heterozygous agouti animals are separable by eye, whereas in nonumbrous mice they are not. Hence umbrous may be regarded as a dominance modifier of agouti.

Umbrous appears to act by controlling the rate or degree of progress of the reaction or reactions whose occurrence is determined by the agouti gene.

Dominance is a character subject to selective control, acting directly through the difference in phenotype of the heterozygote and homozygote, rather than indirectly through a correlation of degree of dominance and variability of the homozygote.