We occasionally get calls from researchers who are using C57BL/6J-bgJ (C57BL/6J-Lystbg-J/J - 000629) mice for the first time asking the following question, "If these mice are called beige, why are they charcoal gray?" The answer has to do with the background strain in which the mutation occurred and was first reported.
First report
The beige (bg) allele, first reported by Kelley, arose at Oak Ridge in 1957 and was probably radiation induced. Kelley describes bg/bg mice on an agouti (A/-) black (Tyrp1b/+, or B/-) background as showing diminished ear and tail pigment and dorsal hairs with the subterminal band yellow, the middle portion dark grey and the base usually very light grey. The overall appearance is cafe-au-lait or beige.
Coat color
Beige-J (bg-J) is a spontaneous mutation which arose at The Jackson Laboratory in strain C57BL/6J. On this non-agouti (a/a) black (B/B) background, bgJ/bgJ mice show diminished ear and tail pigment but the coat color in the absence of the agouti allele is a solid charcoal grey. In a black (B/-) mouse it is the difference at the agouti locus A/- or a/a that makes the coat color appear beige or charcoal grey.
Research
Kelley EM. 1957 Mouse News Lett. 16:36.
Lane PW and Murphy ED. 1972. Susceptibility to spontaneous pneumonitis in an inbred strain of beige and satin mice. Genetics. 72:451-460.