Colorimetric Determination of the Amounts Types of the Guinea Pig.'

H E melanin pigments of mammals fall into two major categories, the T darker pigments or eumelanins and the orange yellow pigments or phaeomelanins. These presumably differ qualitatively although the exact chemistry 01 the melanins is not yet known. The hairs of wild cavies and of the so-called agouti variety of the guinea pig are predominantly eumelanic but have a phaeomelanic subterminal band. Replacement of Eby ee in the genotype of the agouti guinea pig (EABCFP) causes the whole hair to be yellow (at birth) without changing the intensity in the subterminal region. Replacement of Aby aa (with E present) causes the whole hair to be eumelanic without change of intensity of the regions which are eumelanic in agoutis. The combinations with eeaa are yellows (or whites), not distinguishable in the guinea pig a t birth from those with eeA-. In adult animals, a small amount of eumelanin (sootiness) often develops near the tips of the hairs in yellows and this is inhibited in the characteristic subterminal band in the presence of A . There is a second allele of E (namely ep), the presence of which (epep, e*e) results in a tortoiseshell pattern of eumelanic and phaeomelanic areas similar in intensity to colors of corresponding varieties with E and ee respectively, except that some hairs may have mixtures of both kinds of pigment in reduced amounts. Two major kinds of eumelanin, must be distinguished namely sepia and brown, depending in the guinea pig on Band bb respectively (in association with genes which permit eumelanin to develop a t all). A second pair of alleles distinguishes relatively intense (P-) and pale ( p p ) varieties of both sepia and brown. The intensities of all combinations of these genes are affected by a series of five alleles C, ck, cd, c', ca. In albinos (caca) no pigment is present a t birth, though some may develop later under the influence of low temperature. Intensities are highest in most cases with C, and intermediate, but in diverse orders, in the remaining combinations of the C-series. The intensities of phaeomelanin are not affected appreciably by B , b or P , p but are affected by F , f (which have no apparent effect on eumelanin in the presence of P). The C-series effects the yellow as well as the dark pigments. The highest intensities are found with C. There is no yellow in caca, crca and C'C' (in this case even on exposure to low temperatures). The order of effect of the C series differs from any found in the eumelanic series. I n the combination ffpp, replacement of ee by E reduces the intensity of phaeomelanin or wholly