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The B locus is a dilution gene, located on chromosome 4. Dilution genes affect the appearance of other colouration genes. In this case, certain genes at the b locus "dilute" the appearance of black look brown instead.
Dominance
B
Blt
bc
b
2.2.1 B — Black
The dominant B locus is "undiluted": it has no effect on the black pigmentation in the coat. Black agouti mice have yellow and black banded hairs, while black non-agouti mice are black all over.
2.2.2 b — Brown
The recessive b locus dilutes black pigment to a chocolately brown. This occasionally effects the pigment in the eye, making the eye appear more brown than bright black.
2.2.3 Blt — Light
This variation of the b locus has a strong dilution effect. In non-agouti mice, homozygotes appear very pale with brown-tipped hairs, while heterozygotes have dark hairs that are pale at the base. In agouti mice, the yellow bands tend to hide the effect of this locus. It is unknown if this gene is located at the b locus, or if it is epistatic over it[4].
2.2.4 bc — Cordovan
Cordovan is dominant to the brown (b) locus, and displays as a rich deep brown named after a brand of leather.
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