The c locus encodes the gene for the enzyme Tyrosinase. This enzyme is one of a few enzymes involved in the conversion of
the amino acid tyrosine, into the pigmented polymer compound, Melanin.
Mutations of this enzyme have been identified in the rabbit by Aigner
et al(2000) and result in altered levels of melanin
being produced thus effecting the coat colour.
Total inactivation of Tyrosinase results in albinoism, this inactivation can result from single nucleotide polymorphisms (SNP's)
such a mutation has been identified at nucleotide 1118 in the New Zealand White. This SNP involves a C being substituted for an A
in the coding region of the gene.
Other mutations to the c locus lead to variant enzymes which have differing levels of activity. Many of these mutations have normal
enzymatic abilities at low temperatures, however at physiological temperatures of 37 deg C activity is significantly decreased.
The most obvious example of this is the Himalayan allele C
h. Normal tyrosinase activity at low temperatures and no tyrosinase
activity or melanin production at 37 deg C. Less obvious is the dark and light shading genes which produce colours such as Siamese Sable.
The Chinchilla allele has the least effect on tyrosinase activity of the known C locus mutations. The effect on eumelanin production is hard to
determine with the eye. In fact some self black rabbits may infact be homozygous for the chinchilla allele. When the melanins are extracted and
chemically analysed the levels of eumelanin in the hair of chinchillas are lower than the levels from a chestnut agouti. The main effect of the chinchilla
allele is seen on pheomelanin production. This results in the removal of all the yellow from a chestnut agouti producing the highly attractive Chinchilla
phenotype.
Epistasis
The phenotype seen in the absence of tyrosinase is albino. This phenotype is unchanged no matter what other colour mutations are seen so in effect the
"c" allele blocks the phenotype of genes at the agouti, brown, dilute and extension loci.
C locus alleles
C - Full colour, complete tyrosinase activity
C
ch3 - Chinchilla, Reduced eumelanin, eliminated pheomelanin production.
C
ch2 - Dark Shadding
C
ch1 - Light Shadding
C
h - Himalayan, Temperature sensitive mutation
c - albino, Inactive tyrosinase
References
Aigner B., Bessenfelder U., Muller M. and Brem G. (2000)
Tyrosinase Gene variants in Different Rabbit Strains Mammalian Genome
11, pp700-702
