p is a very old mutation carried in many varieties of fancy mice (J:12958). It has been suggested that the original mutation occurred in Japanese wild mice, Mus musculus molossinus (J:19782).
Homozygotes have pink eyes with pigmentation very much reduced but not completely absent in both the retina and choroid. The black pigment of the hair is very much diluted, but the yellow pigment is only slightly affected. Pigment granules are irregular and shred-like in shape. The small amount of pigment they contain is of wild-type color (J:12970, J:12958). The fine structure of the pigment granules was said by Moyer (J:5001) to be disrupted, but Hearing et al. (J:5346) found the structure to be normal, with premature termination of the melanization process.
In tissue culture of the eye, the amount of pigment formed can be increased by increasing the concentration of tyrosine. This suggests that p may block the melanin-synthesizing pathway by interference with tyrosine supply (J:12726). The site of gene action is in the melanocytes and not in either the dermis or the epidermis (J:7988).
A presumed p gene has been cloned (J:2206). It was isolated from mouse melanoma and melanocyte libraries and is missing or altered in six independent p mutant alleles (J:2206). By sequence comparison, the human P locus, deletions of which are associated with hypopigmentation, is orthologous to p (J:2206). P maps to Chr 15q, near the Prader--Willi syndrome locus. On the basis of this location, the p mutation has been proposed to provide a mouse model for Prader--Willi syndrome, for Angelman syndrome, for one form of hypomelanosis of Ito (J:3253), and for type II oculocutaneous albinism (J:3600). A small nuclear ribonucleoprotein particle gene Snrpn maps near p and its human ortholog in the homologous Prader--Willi region of human Chromosome 15 (J:3623). Snrpn appears to be a better candidate for the Prader-Willi syndrome ortholog. P is deleted in human type II oculocutaneous albinism, making p a model for this disease (J:3600).