Locus C - Colour Point restriction (Siamese, Burmese allele)

The alleles of C-locus belong to the so-called Albino series: C (full colour) > c^b (Burmese) = c^s(Siamese) > c (complete albino). The TYR-gene codes for enzyme tyrosinase that is necessary for melanine production. I n this gene, mutations have been identified that are responsible for pigment production causing the coat colour, which is typical in Siamese and Burmese cats (the pigmentation being restricted to colder body extremities (mask, limbs and tail), other body parts are very pale or white).

The original (wild type) allele called C is dominant to c^b-allele (Burmese) that is incompletely dominant to c^s-allele (Siamese). The C-allele produces the full colour, i.e. the maximum possible degree of pigmentation.

The mutation c.901G˃A (p.G301R) resulting in c^s-allele (Siamese) causes reduction of coat pigmentation and colour change to Himalayan or Siamese. In individuals with c^s/c^s-genotype, the phenotypic expression results in colour reduction and production of marking. The eyes are blue.

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The c^b-allele (Burmese) is a result of mutation c. 679G˃T (p.G227W) and reduces partly the intensity of the pigmentation to Barmese colour. It is a coloration with marking, while the body colour is only slightly paler. The colour of black cat changes due to presence of  c^b/ c^b-genotype to brown colour, so-called sepia brown. The eyes are amber or yellow.

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Individuals with c^s/c^b-genotype are intermediate in colour - mink.

Genotypes:

• C/C full colour, the individual is neither a carrier of c^s-allele (Siamese) nor c^b-allele (Burmese)
• C/c^b full colour, a carrier of c^b-allele (transmits the so-called sepia brown colour)
• C/c^s full colour, a carrier of c^s-allele (a carrier of a colour typical in Siamese cats - so-called colour point restriction)
• c^b/c^b - an individual with Burmese colour (homozygous for c^b-allele (Burmese), an individual transmits this allele to all its offsprings
• c^s/c^s - an individual with Siamese pattern (homozygous for c^s-allele (Siamese), this individual transmits c^s-allele to all its offsprings)
• c^b/c^s- - an individual with mink colour

The molecular-genetic test also detects potential carriers of c^s-allele (Siamese) and c^b-allele (Burmese).

The rare c^a allele of TYR-gene resulting in a phenotype with white coat colour and blue eyes cannot be detected by this test.

In connection with the albinism, the cytosine deletion at position 975 in exon 2 in TYR-gene was described. Albino individuals with blue eyes were included in the study (Imes et al. 2006).  Other genes of so-called W-locus also participate in the white coat colour and in this case the white colour is determined by different genes and not by TYR-gene.

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References:

Lyons, L. A., I. T. Foe, H. C. Rah and R. A. Grahn, 2005 Chocolate coated cats: TYRP1 mutations for brown color in domestic cats. Mamm. Genome 16: 356-366.

Schmidt-Küntzel, A., E. Eizirik, S. J. O'Brien and M. Menotti-Raymond, 2005 Tyrosinase and tyrosinase related protein 1 alleles specify domestic cat coat color phenotypes of the albino and brown loci. J. Hered. 96: 289-301.

Imes, D. L., Geary L. A.,  Grahn R. A., Lyons L. A. 2006 Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation, Amimal Genetic, 37, 175-178