Although we have already considered sombre ( Eso) and tobacco
darkening ( Etob), alleles of the extension locus which cause
darkening of the coat, there are a number of other unrelated
determinants which also produce similar effects. Thus, as
mentioned previously, yellow ( Ay/;B/) mice are often
characterized, especially in outbred populations, by variable
degrees of sootiness. In some animals the sootiness is confined
to a mid-dorsal streak, in others this streak is wider, covering
the entire back and sometimes the flanks, so that only the belly
is phenotypically "yellow." This situation is due to the
admixture of hairs possessing significant amounts of eumelanin
to the yellow fur. If these hairs are relatively few, the animal
appears to be dingy yellow; with increasing numbers, however,
the dorsum becomes darker until, at the extreme, an animal with
a dark sable back and yellow belly is produced. 39 As in the
case of Eso and Etob, these modifications are not limited to
animals of yellow genotype as the amou