California blonde deer mice have diluted tan pelage, unpigmented skin and ears and reduced pigment in the retinal epithelium. The ear and tail stripe pigmentation is dilute tan in contrast to the darker color of the wild-type. The eyes, when examined with bright light, have a distinctly reddish-brown color, in contrast to the intensely dark eyes of the wild-type. Microscopic sections through the optic cup, as observed in neonates reveal a lack of pigment in the pigment layer of the retinal epithelium in the California blonde mutant. The California blonde phenotype is very similar to that of the brown, blonde and golden nugget mutations in Peromyscus.
The light tan dorsal coat color of California blonde is due to brown eumelanin granules in the individual hairs replacing the darker, near-black normal pigment granules, as seen upon microscopic examination of individual hairs. This condition is shared with other other blonde and brown-like mutants. The granules in the California blonde hairs are somewhat darker than those of blonde, but lighter than those of brown and golden nugget. In California blonde and blonde the base of the hair is a pale grayish, whereas in brown and golden nugget the base is very pale tan and in platinum the hair bases are virtually unpigmented.
Source: In 1981 a tan-colored male deer mouse appeared in a breeding stock of P.m. santacruzae derived from wild ancestors collected on Santa Cruz Island CA, during 1978 and 1979. The trait was designated "California blonde" to distinguish it from the previously named blonde mutant.