Colour genes

Marking genes


Officially known as brown (hence it being the B locus), we tend to call the effects of this gene chocolate in rats. It’s a fairly old gene, first being described by Castle and King in 1932. The alleles at this locus are:

  • B – Dominant wild type (so “no effect”)
  • b – Recessive brown gene. Black pigment becomes deep chocolate brown. Yellow pigment is not changed.

In other fancies both light and dark brown mutations have been recorded here, so maybe we have more shades of brown to look forward to in future.


aaB-: Black

aabb: Chocolate

A-bb: Chocolate agouti