Mi-spotted (misp) is a new mutation in the mouse at the microphthalmia (mi) locus. It has no obvious visible effect in either the heterozygote (misp/+) or homozygote (misp/misp) and was discovered by virtue of its interaction with white (miwh). Mice of genotype miwh/misp are pale yellow with white spots; yellow areas later become ‘sooty’ at the first moult.
Though mice of genotypes a/a; misp/+ and a/a; misp/misp cannot be visibly distinguished from those of a/a; +/+, the amount of tyrosine-2-C14 incorporated in melanin in skins of 5-day-old mice of these genotypes differed. Assays of tyrosine incorporation were extended to include other non-agouti genotypes differing only at the microphthalmia locus. The amount of tyrosine incorporated was greatest in +/+ followed in order by misp/+, misp/+, misp/misp, miwh/+, miwh/misp, and miwh/miwh.
Pigment granules were examined in club hairs of these same genotypes for different regions of the hair shaft. Hairs of misp/+, misp/misp, and mi/+ could not be distinguished from +/+ either in number, kind or arrangement of granules. Hairs of miwh/+ showed reduced cortical and medullary pigment, especially distally, which was even more pronounced in hairs of miwh/misp. Medullary granules of miwh/misp varied greatly in size with a few large yellow-brown granules.