Variable White (Vw)
ALLELES: Vw (mutant), Vw+ (wild-type).
INHERITANCE: Semi-dominant lethal, variable expression.
PHENOTYPE: Considerable variation occurs among individuals in the extent of unpigmented pelage. There is extensive white on the face, ears and forehead. The tail is largely unpigmented in most individuals, and invariably is white at the tip. In the unpigmented areas the individual hairs are white throughout. The mid-dorsum is nearly always pigmented, but not clearly delineated. The margins of the pigmented areas appear ragged and uneven. Furthermore, the pigment is less intense or more diluted than in wild-type hairs, giving the pigmented regions a grayish-tan wash. The eyes appear dark like those of the wild-type.
Microscopic analysis of fixed, paraffin embedded, stained sections of skin revealed an absence of unpigmented melanocytes (clear cells) in hair follicles from the white areas of the skin, while pigmented melanocytes were present in the pigmented portions of the pelage.
A noticeable feature of the mutant phenotype is variability in response to auditory stimuli. Some individuals appear to be deaf, while others react normally. Auditory deficiency was more pronounced in the males tested. In general, animals withe high expression of white on the head were more likely to exhibit deafness, indicative of a basic defect in neural crest cell migration.
Female variable white usually do not bear viable young.
Variable white deer mice are unable to swim.
SOURCE: The first variable white mutant animal appeared spontaneously in 1983 in a long-established stock of wild-type prairie deer mice, P. m. bairdii, maintained at Michigan State University. A mated pair which had previously yielded more than 50 wild-type progeny produced a single female exhibiting the variable white phenotype.
REFERENCE: Cowling et al. (1994).