A   gene   at  one  locus  may  affect
   phenotypic expression of a gene at another locus by epistasis.
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   The  B/b  gene  determines  the  pigment  color (B for black and b for
   brown)
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   The  epistatic  C/c  gene  controls whether or not any pigment will be
   deposited in the hair.
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   A  homozygous  recessive  cc  mouse  has no hair pigment and is albino
   regardless of its B/b genotype.