Chapter 5

Grey-Lethal, Grizzled, Mocha, Pallid, Muted, Misty, and Pearl

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II. Grizzled (gr)

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

"Grizzled" ( gr; chromosome 10) is similar to grey-lethal ( gl) in that it too is a recessive condition which influences viability and phaeomelanin synthesis ( Bloom and Falconer, 1966). In fact, the only reliable phenotypic effect of this mutation is that it dilutes yellow pigment.

A. Influence on Coat Color

A/A;gr/gr mice look much like chinchilla ( A/A;cch/cch) animals except that the dilution is usually more pronounced and, unlike chinchilla, eumelanin synthesis is unaffected. Grizzled is recognizable also in nonagouti ( a/a) mice by the hairs on the ears and around the genitalia appearing white instead of yellow. When combined with yellow ( Ay) grizzled produces a less pale color than does chinchilla but only because of the persistence of sootiness (eumelanin) in the coat.

The fact that gr and gl are situated about 20 cM from each other is undoubtedly a coincidence. Nevertheless, it would be of interest to know whether the dilution of yellow pigment in grizzled animals occurs via a clumping of granules, as is the case with grey-lethal, and/or whether it is due to a general reduction in the number of granules. The interaction of gr with recessive yellow ( e/e) also is germane.

B. Other Manifestations

Grizzled mice are about three-quarters the weight of their normal littermates at birth and, although they subsequently grow relatively a little faster than normal, they are a little smaller when adult. They are also less viable, both prenatally and postnatally, than their normal littermates with the mortality being higher in males than in females. The prenatal mortality occurs at all stages of development from about 10 days to after 18 days. Some grizzled mice also have kinky tails, the degree of kinkiness being very variable ( Bloom and Falconer, 1966). The cause of the "grizzled syndrome" is not known.

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