Color and Pattern Genotyping in Sphynx

The term genotyping refers to the specification of the two alleles for each of set of coat color genes which determine the phenotypic appearance of a sphynx or any other cat.  Genotyping can be done for other features of the cat which display simple Mendelian inheritance (e.g. hairlessness in the sphynx or curled ears in the Scottish Fold); however, only the coat colors will be discussed here.

The nine major coat determinants are agouti/non-agouti (A/a), black/chocolate/cinnamon (B/b/bl), color/points (C/cb/cs/ca/c), dense/dilute (D/d), pigment inhibitor/normal color (I/i), orange/normal color (O/o), white spotted/solid (S/s), tabby pattern (Ta/T/t) and solid white/non-white (W/w).  The upper case letters correspond to the dominant alleles at these genetic loci while lower case indicates a recessive locus--some alleles show co-dominance (e.g. ticked and mackerel tabby patterns) with both being represented with upper case letters while others show increasing recessiveness (e.g. chocolate and cinnamon coat colors) with both being represented in lower case letters.

The dominant agouti allele (A) codes for the presence of tabby markings seen on cats while the recessive non-agouti allele helps to hide these markings.  However, a homozygous non-agouti cat (aa) which is red or cream may still show these markings.   The tabby alleles (Ta/T/t) help to determine what type of tabby markings the individual cat may have.  Mackerel tabby markings (T) are dominant genetically and produce tiger-like stripes on the cat while Abyssinian type tabby markings (Ta) are co-dominant to the mackerel type and produce ticking--stripes occur on the face and tail if the (T) allele is present with the (Ta) allele.  A third type of tabby pattern, blotched markings (tb), produce the classic tabby pattern phenotype.  The spotted tabby pattern is a form of the mackerel tabby markings and is the result of incomplete expression of the mackerel stripes and/or blotched tabby pattern--the absolute genetic determinance of the spotted markings in relation to the (T) allele is uncertain.  With respect to the sphynx, one should note that the relative hairlessness of this breed makes the coat expression of the ticked tabby pattern an impossibility while the spotted tabby pattern tends to slowly blend into the cat's coat color as the cat ages.

Other alleles that code for adjustments to the pattern of the solid coat color are those for color points (c) and piebald spotting (S).  The color point alleles are recessive in nature and can produce varying degrees and types of coat features.  The major types of color point alleles are those of the Burmese (cb), the Siamese (cs), blue-eyed albino (ca) and pink-eyed albino (c).   These alleles require homozygosity in order to be expressed and do so through varying degrees of melanin production which is inversely to the temperature of the particular body site.  Homozygous Siamese color point alleles yield a coat which is lighter with darker points (on the ears, feet and tail) and blue eyes (phenotype = "pointed"); homozygous Burmese color point alleles yield a coat which is darker (in comparison to the Siamese) and gold eyes (phenotype = "sepia"); one each of Siamese and Burmese color point alleles yield a Tonkinese appearance with blue, aqua or gold eyes (phenotype = "mink"); homozygous blue albino point alleles yield a white coat and blue eyes while homozygous pink albino point alleles, extremely rare, yield the white coat with pink eyes.  In all of these pointed phenotypes, the kittens are born white with color filling in over time depending on the type of point alleles the cats has.  The piebald spotting allele (S) is a dominant one which brings the "tuxedo," "gloved" and "Van" type phenotypes to a cat.  There are varying levels of expression for this allele, and it is well described that some spotting alleles are very strong (i.e. Van--minimal coloring other than white on body except one to two colored spots usually on head) while some leave little to no visible spotting.  Some geneticists have described Vans to be homozygous for this allele; however, there are Van phenotypes derived from cats heterozygous for this allele and there are cats which are homozygous for this allele while not showing the Van phenotype.

The solid color of the coat is determined by the combination of at least five gene systems:   black/chocolate/cinnamon (B/b/bl), dense/dilute (D/d), orange/normal (O/o), piment inhibitor/normal (I/i) and solid white/normal (W/w).

White

The solid white allele is a dominant one which hides all other colors of the cat's coat--sometimes the kitten may show some slightly colored hairs, but these usually disappear with age.  Associated with this coat color is the presence of none, one or two blue eyes.  This trait is also associated with deafness in one or both ears with blue-eyed, white cats having a 50% prevelance of deafness and gold-eyed, white cats with a 20% incidence--one should remember that blue-eyed, white cats can also be produced through homozygosity of the blue-eyed albino point gene, so a history of the cat's coat inheritance is important in absolute determination of the genotype in this particular feline phenotypic appearance.  The pigment inhibitor gene is another dominant gene which inhibits other coat colors; however, it produces a silver or chinchilla ticked coat coloring--and also cannot be appreciated in the extremely hairless sphynx cat.  Another solid coat color gene is the dominant orange gene which turns all coat color to orange and is situated on the X chromosome.  Since males only have one X chromosome, any male with the orange color gene will be red or cream.

Red

Red or cream females must be homozygous for the orange color gene even though it is a dominant gene.  The reason for this is that one X chromosome in each female cell is inactivated and does not express its genes.  Because this is not always the same chromosome in every cell, heterozygous females display a mosaic pattern called the tortoiseshell pattern which is a mix of the red/cream color and the black/blue/cinnamon/chocolate/lilac/fawn color.  Interestingly, the red/cream coated cat can be heterozygous for the non-agouti gene (aa) and still can display the tabby pattern--through selective breeding, red/cream cats can be made to "lose" their markings with this being more difficult to acheive in the face, legs and tail.

Black

The black gene is a dominant allele which allows expression of coat melanin to appear as black if the orange gene is not present.  The chocolate allele on this same locus is recessive to the black allele, and two of these produce a chemical change in melanin and a  chocolate (brown) coat appearance--the cinnamon gene is also an allele on this locus, is recessive to the chocolate gene and produces an even lighter brown coat color when both alleles are present. This melanin is further affected by the dilute gene with homozygosity leading to decreased melanin production and change of black coats to blue, chocolate coats to lilac, cinnamon coats to fawn and red coats to cream.  These coat colors often can be very difficult to discriminate with some dilute colors falling in between these "standard" colors and determinations sometimes being made by looking at the paw pads and nose leather (dark for non-dilute cats, pink for dilute cats).

A summarization of the activities and interactions of these nine coat color genes is shown below:

Rules Governing Phenotypic Expression of Feline Coat Colors

1.  One white coat allele will make any cat's coat solid white

2.  One pigment inhibitor allele without a white coat allele will make any cat's coat solid silver ticked

3.  Homozygous orange allele carriers without white or pigment inhibitor alleles will make a cat's coat solid red

4.  A lack of white, inhibitor or orange alleles will give a cat a dark coat color

5.  Specific dark coat colors are characterized by the alleles present for black/blue/cinnamon and dense/dilute gene sites

Exceptions to Rules Above

1.  Homozygous blue-eyed or pink eyed color point allele presence will yield a white cat no matter what other alleles are present

2.  Homozygous Siamese or Burmese color point alleles will lighten coat color over warmer body areas only

3.  White spotting alleles will yield discontinuity in solid coats in a variable manner

4.  Slight patches of color can sometimes be visualized with the white coat allele--this usually disappears with age

5.  At least one Agouti allele with homozygous ticked tabby alleles in a red or dark cat will lighten the end of the hair shaft (ticking)

6.  At least one Agouti allele with one ticked tabby allele and one mackerel tabby allele in a red or dark cat will yield ticking and facial/tail striping

7.  At least one Agouti allele with homozygous mackerel tabby alleles in a red or dark cat will yield body striping

8.  One or two mackerel tabby alleles in a red cat may yield facial, tail and/or body striping

9.  One Agouti allele with homozyous blotched tabby alleles in a red or dark cat will yield the classic tabby phenotype

Rules Governing Dark/Dilute Coat Phenotypes

1.  One black coat allele will keep a dark coat black

2.  Homozygous chocolate coat alleles or one chocolate allele and one cinnamon allele will make a dark coat chocolate

3.  Homozygous cinnamon coat alleles will make a dark coat cinnamon

4.  Homozygous dilute alleles affect solid coat colors in the following manners:

a.  Black coats become blue

b.  Chocolate coats become lilac

c.  Cinnamon coats become fawn

d.  Red coats become cream

Rules Governing Tortoiseshell Coat Phenotypes

1.  Any cat with multiple X chromosomes and at least one each of the orange allele and normal (non-orange) allele will have a tortie coat

2.  These coat phenotypes are subject to all of the above rules governing color and pattern expression

 

~ This article published with permission from it's author, Dr. James Thoene of Senspelo Sphynx.