Over the past decades scientists have shown that cats, mice and humans share similar genetic systems. This has been achieved by decoding the DNA that underpins the genes and chromosomes of many species and finding large stretches of DNA homology between them. Like other mammals such as dogs, sheep and cattle, the DNA of the cat is made up of a universal four base code signified by the letters A, T, C and G. It has been estimated 3 billion of these letters make up the 38 chromosomes of the cat. Mutations in genes occur when the DNA is altered. Changes in DNA might be as small as a single letter or it may involve the deletion of many letters or the replacement of one letter with another. Such mutations may happen when DNA is disrupted by a virus; damaged by UV light or chemicals. Either way there is a misspelling of the DNA alphabet and there may be no gene product or an altered gene product generated. Unfortunately, some DNA mutations may lead to harmful heritable diseases or genetic defects, many of which result in analogous genetic syndromes in humans, laboratory mice and other mammals (reviewed by Fowler et al, 2000). Apart from being deleterious, changes in the DNA blueprint are also responsible for the amazing variety we see in any species including the great diversity of cat breeds and coat colours that can be viewed today.

For example, different recessively inherited mutations in the DNA of the black colour gene of non-agouti (gene symbol: a) cats may give rise to brown or chocolate, or cinnamon coloured cats. When the black gene (B) or seal brown (genetically black under the influence of the Burmese or Siamese colour genes known as c^b and c^s, respectively; Vella et al, 1999) is exposed to the recessive dilute density of pigment gene (d), blue coat colour (aaBBdd) is observed. Conversely when chocolate brown (b) and the dilute gene are co-inherited lilac (aabbdd) is noted, alternatively when cinnamon or light brown (b¹) pairs up with the dilute gene, fawn or light cocoa brown (aab¹b¹dd) appears. When dilute coat colours of cats including fawn, are exposed to the dominant dilute modifier gene (Dm) a range of coat colours known as caramel (blue-based), taupe (lilac-based) and apricot (cream-based; cream being the dilute density of the sex-linked red gene known as the gene symbol, O) can be generated (reviewed by Vella et al, 1999). However, while this brief summary of dilute coat colour inheritance gives cat fanciers a good foundation for understanding the genetic basis of dilutes, it does not explain the pale kaleidoscope of colours that may be observed within a particular dilute group. It is highly plausible that the answer to some of the curious coat colours seen in cats resides in the DNA code of the genes responsible for coat colour.

Unlike the cat where very little is known about the precise spelling of the DNA that encodes coat colour genes, many colour genes including the dilute gene have been identified in the mouse and their DNA sequence determined (Mouse Genome Database, 2000) . Over the decades, lots of subtly graded shades of coat colour have been associated with various forms of mutations in the dilute gene of mice. Indeed at least 17 types of mutations have been individually characterised at the molecular DNA level in different regions of the mouse dilute gene. These mutations result in the dark coat colour of normal mice being diluted to an array of coat colours that vary from medium to light in colour (Davis and Justice, 1998). Interestingly some of the different versions of the mouse dilute gene not only result in a change of coat colour pigmentation but may also produce a range of neurological impairment symptoms (Davis and Justice, 1998). This observation is perhaps not surprising given that several mutant gene products from other genes that impact on pigmentation (eg white spotting genes) have been shown to have a dual role in affecting the normal development of pigment and nerve cells in mammalian embryos (Mouse Genome Database, 2000).

Overall, the findings in mice suggest that at the DNA level some cat breeds or breeding lines, may have a slightly different spelling in the DNA sequence of their dilute gene. Moreover the mouse studies imply that altered forms of the feline dilute gene may also be responsible for some of the subtle variations that can be observed in individual cats displaying one of the recognised dilute colours. Theoretically the Feline Genome Project that has begun decoding the genes that make up the cat has the potential to answer many of the tantalising curiosities that surround the inheritance and expression of coat colour genes in cats (Fowler, 1998). However, without special funding and resources this task remains in the distant future.

Meanwhile the big challenge for cat breeders, judges and registering bodies is to decide if it is acceptable to classify the fawn-based, lilac-based, blue-based, Dm-influenced coat colours as well as other dilutes that may possibly exhibit ‘fawning’ tones due to a special form of mutation in the dilute gene or yet to be defined polygenes, under the umbrella label of ‘caramel’. The UK foundation breeder of caramels, Patricia Turner suspects that the differences between fawn-based, lilac-based, blue-based Dm-affected coat colours are too elusive to classify as separate categories (Turner, 1992). However Robinson’s current text draws a clear genetic and phenotypic distinction between the blue-based caramel (aaBBddDm-) and the lilac-based taupe (aabbddDm-) stating that ‘Blue under the influence of this mutation (Dm) takes on a brownish cast, but does not become as light in tone as the lilac’ (Vella et al, 1999).

Classifying ‘caramel’ cats by phenotype alone has invariably lead to no end of confusion for present fanciers and control bodies. This route also leaves the door open for otherwise non-classifiable, dilute cats to be placed in the ‘caramel’ category despite no prior ancestry of the Dm gene. In the absence of knowing the precise DNA sequence that encodes the coat colour genes of such cats, test mating for the presence of the dominantly inherited Dm gene, or an unusual version of the recessive dilute gene may be one way of resolving whether a cat’s colour is truly Dm-affected. However breeders need to keep in mind that the Dm gene has no apparent effect on non-dilute coat colours (Turner, 1992; Vella et al, 1999).

Alternatively, by taking genotype into consideration when categorising feline colours, fanciers can exploit the clues that can be found in the cat’s parentage. If registration bodies choose the direction offered by Robinson’s latest book thereby restricting ‘caramel’ to blue-based Dm-affected cats, then pedigrees with blue or lilac heritage can be a valuable asset in defining blue- or lilac-based, Dm-cats as caramels or taupes, respectively (Vella et al, 1999). To help clarify this issue, governing bodies may look to past policies with regard to classification of coat colours. However this may not truly resolve the path that councils adopt as discrepancies in colour nomenclature policies already exist. For instance, the Australian Mist colours known as gold and peach are clear examples where the genotypic differences between gold/cinnamon and peach/fawn resulted in gold and peach colours being recognised in their own right (Straede, 1992). Likewise the contribution of Burmese colour genetics (c^bc^b) that lighten chocolate or lilac self coat colours have been acknowledged in some countries by registering these coat colours as champagne and platinum, respectively (Brown, 1992). However elsewhere genotypic disparities between chocolate/champagne and lilac/platinum coat colours remain quietly unappreciated with some countries classing these Burmese colours under the blanket names of chocolate or lilac.

Nonetheless, despite the outcome of the registration process, at some stage judges will be called upon to decide whether cats entered in the ‘caramel’ class fit the written description for caramel. The show standard for caramel Oriental cats in the country of origin asks for ‘Cool toned bluish fawn, coloured to the roots. No white hairs’ (GCCF, 2000). If the judges, ‘being the keepers of the standard’ comprehend this concept and adhere to it, then the cats that fit this criterion are the ones that are likely to be commended on coat colour thus helping breeders and fanciers decide what flavour of caramel is the correct one.

References

D Alderton (1992)
Short-haired cats in Cats
Dorling Kindersley Ltd, London
Harper Collins; Pymble, NSW, Australia

D Brown (1992)
Feline Colour Genetics
Dearinger Enterprises; Utah, USA

AP Davis and MJ Justice (1998)
Mouse alleles: if you have seen one, you haven’t seen them all.
Trends in Genetics, 14: 438-440

KJ Fowler (1998)
What can mice teach us about cats?
Feline Focus, 26 (2): 7
Available at: http//www.hotkey.net.au/~fccvic/art4.htm#home

KJ Fowler, MA Sahhar and RJ Tassicker (2000)
Genetic counseling for cat and dog breeders – managing the emotional impact.
Journal of the American Veterinary Medical Association, 216: 498-501

GCCF Standards of Points for Oriental Cats
Available at: http://www.palantir.co.uk/osop.html
Accessed: April, 2000

Mouse Genome Database
Available at: http://www.informatics.jax.org
Accessed: August, 2000

TM Straede (1992)
…And they come in six colours.
National Cat July, 20-21

P Turner (1992)
The Birth of Caramels.
Courtesy: D Turner, NSWCFA

CM Vella, LM Shelton, JJ McGonagle and TW Stranglen (1999)
Colour Inheritance in Robinson’s Genetics for cat breeders and veterinarians.
4^th edition, Butterworth Heinemann; Oxford, UK

Acknowledgement

The author thanks cytogeneticist Lucille Voullaire for commenting on this manuscript and the Coordinating Cat Council of Australia for the opportunity to present this paper at the recent judges conference held in Sydney. Kerry Fowler is Head of Disease Model Unit, a Senior Research Officer with Murdoch Childrens Research Institute, Royal Children’s Hospital, Flemington Road, Parkville, Victoria, 3052 and an Associate Research Fellow with Department of Paediatrics, University of Melbourne.