I case: Coat colour in rabbits
Coat colour in rabbits is controlled by a single gene which has its four different versions or alleles. That is why there exist not only the white and black coloured rabbits but other coloured as well.
II case: C series in dogs
Gene C is required for colour but a recessive cc yields an albino in dogs. A dog with c gene cannot produce tyrosinase and melanin pigment. Gene C has many alleles which control the coat colour in dogs. CC dogs expresses the colour series while cc are albino with pale eyes and nose.