Chapter 6

Beige, Silver, Greying with Age, and Other Determinants

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III. Greying with Age (Ga)

For the Ga allele:
Ga Allele (MGI) Gene (MGI) All Alleles (MGI)

Greying with age ( Ga), an autosomal dominant, produces a phenotype similar to silver. The mutation was found in wild mice from several localities in South Australia by Kirby and the following is based on his account ( Kirby, 1974). Ga has not been mapped nor have crosses been made to rule out the possibility that it is allelic with si.

Two wild grey mice, one from Yorke Peninsula and one from Adelaide, were bred in Kirby's laboratory. Although the stocks derived from these animals have been maintained separately, animals of both stocks look alike and it is assumed that the greying is produced by the same factor.

A. Maternal Influence

Unlike silver, greying with age does not manifest itself until relatively late in life. Moreover, its expression seems to be influenced by maternal effects. When Ga/— mice were crossed reciprocally with C57BL ( ga/ga) animals, different results were obtained. When the female parent was C57BL, the proportion of grey mice in the progeny was considerably lower than when the male parent was C57BL. Only 7 of 65 of the progeny of 8 C57BL females were detected as being grey after they were more than 10 months old, indicating that more of these offspring undoubtedly would have become grey if they had been maintained longer. On the other hand, all 12 of the progeny derived from 3 Ga/— females became grey in less than 10 months. Indeed, the fact that all of these progeny became grey indicates that one or more of the mothers was probably homozygous for Ga, and that such animals are viable and fertile.

B. Influence of Genetic background on Expression

The age at which Ga/— mice begin to lose their hair pigment is variable and depends upon their genetic background. In animals which had been selected for three generations for early greying it was noticeable as early as 3 months of age, whereas it did not appear until after 1 year of age in animals outcrossed to C57BL.

Ga/— mice on an a/a;B/B background become grey first on their belly, then usually on their face and hind quarters. The last area affected is the mid-dorsum. Furthermore, the greying is variable; some mice are only faintly grey while others are nearly white. Microscopic examination of Ga/— hairs revealed most of them to be either fully pigmented or completely lacking in melanin; only rarely were partially pigmented hairs found.

C. Comparison with Silver and Other Considerations

Although there is a marked resemblance between greying with age mice and silver mice, Ga differs from si in that it is inherited as a dominant, is recognized later in life, produces fewer mosaic hairs, and is influenced by maternal factors. The etiology of this condition remains to be elucidated. Nevertheless, the observation that, unlike silver, the greying intensified with age in the presumably agouti (or white bellied agouti) wild population in which it occurred suggests that it may have a different etiology. Information on its expression in Ay/— animals as well as whether repeated plucking can hasten its expression is awaited.

Ga is also of interest because it evidently has been present in widespread local populations of wild mice in South Australia for some time. Its exact frequency in the wild is difficult to establish because most Ga mice in the laboratory do not express the gene until about 6 months of age and few wild mice reach this age (Newsome 1969a, 1969b). Hence, one wonders what the selective forces, if any, are for maintaining this polymorphism?

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