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Simple Genetics


For Cat Judges & Breeders


How do you know when a white cat is really a tabby point? Simple-you look up his jeans-sorry, genes! Most people turn off when you start talking about genetics. You can tell because their expression becomes glazed, and they get that 'out to lunch' look in their eyes. They change the subject at the first opportunity, nod off or wander away. It is not necessary to have a university degree to understand simple genetics-you just have to be interested-the more you learn about the subject, the more fascinating and relevant it becomes. A basic knowledge of genetics is of vital importance to us as judges and cat breeders.


As a judge, you will be consulted by breeders, exhibitors and owners who may consider you to be an expert on everything pertaining to the breed you are licensed to judge-and that includes genetics. Understanding how genes affect the different breeds and colors will help you become a more informed judge and you will find it easier to be flexible in your judging.


On the judging floor, judges may be tempted to apply their own breeding desires or prejudices about certain breeds, particular features or colors-however, they must be completely impartial and unbiased. Of course, one judge can never keep all the breeds, or even all the varieties within in each breed, but we can talk to the breeders and go to see their cats and kittens at various stages of development. You can also read as much as you can on the subject-Roy Robertson's Genetics for Cat Breeders is an excellent reference book, and Gloria Stephens' Legacy of the Cat has a very readable section, also The Book of The Cat. It is essential to keep an open mind and keep learning.


As a breeder you need to know how to plan your breeding program, what colors to expect, and how inherited problems can be avoided. The following are some simple explanations of terms you probably have heard before but may not have understood, starting with definitions:

Gene: (from the Greek genos origin) the hereditary factor transmitted by each parent to offspring and which determines hereditary characteristics.


Genetics: the scientific study of the heredity of individuals, especially of inherited characteristics.


Genes: Every living being that is reproduced from two parents inherits characteristics equally from both of them. These characteristics are determined by genes, control mechanisms carried rather like beads on strings along two rod-like bodies, called chromosomes. For each particular trait or characteristic, there is a gene arranged in a particular order along the chromosome that controls the expression of that trait.


Cells and Chromosomes: Living organisms are composed of cells. A typical cell contains a nucleus within which are DNA and RNA-the building blocks of life. The DNA is organized into chromosomes which in turn carry the genes.

There are two types of cells, body cells and sex cells, and in the cat each cell has 38 chromosomes, which are arranged in pairs-19 pairs in all. Sometimes both halves of a pair carry identical genes, sometimes not. Out of these 38 there are only two chromosomes that determine the sex of the individual-the X and the Y chromosomes. Only males have the Y chromosomes, and XYXX. Prior to fertilization taking place at mating, the gametes (sperm or ova), receive half a set of chromosomes from each parent. During fertilization, the sperm and ova fuse to produce a new genetic combination in the resulting fertilized cell or zygote. The newly formed zygote contains a random selection of those genes the parents have inherited from their parents. These then combine in the new cell to make up the full complement of 19 chromosome pairs. A zygote develops into an embryo/foetus/newborn by cell division called mitosis. In normal cell division, which creates new cells for growth, the full set of genes is replicated for each new cell. denotes a male, while the female is.


The animal's genetic makeup is called its genotype. Some of these characters may be hidden and are not perceived when one looks at the cat. The outward appearance is the phenotype. The difference can be caused by dominant or recessive genes. There are also genes that are not fully penetrant, masking genes, modifier or polygenes, sex-linked genes, and inhibitor genes, as well as disease-causing genes-but more of these later.


Modifiers or Polygenes, although not individually having a great effect, can act together to modify major genes such as those controlling the length of coat and depth of color in fur or eyes.


Homozygous & Heterozygous: If a kitten receives identical genes from both parents for a particular characteristic, i.e. the genes on that pair of chromosomes are the same, and the animal is said to be homozygous for that trait. But if it receives a particular gene from one parent and an alternative from the other, the pair is made up of two different genes, and the animal is said to be heterozygous for that characteristic. Genes with a matched partner for a comparable characteristic-hair length for instance-are called alleles, and are found in the same spot or locus on the chromosome.


Once the basic rules of inheritance are understood, and the dominance or recessivity of any specific characteristics have been determined, it is possible to work out the characteristics to be expected from virtually any crosses between cats whose ancestry and genetic makeup is known.


Dominant and Recessive Genes: If a cat is homozygous (BB or bb) the same message is sent and received, both genes on that pair of chromosomes are identical. If it is heterozygous (Bb) the dominant gene is in control-the recessive (b) is still there, but may have little or no effect over (B) its dominant partner. Solid white is a masking gene and is dominant to all other colors; black (or seal, or brown) are dominant to light brown and chocolate; tabby (agouti) is dominant to self or solid (non-agouti); shorthair is dominant to longhair, to mention just a few.


A Seal Point Siamese, for instance, is homozygous for the recessive Himalayan (or Siamese) pattern, but it may also be heterozygous if it carries the recessive gene for chocolate. Recessive genes may be 'carried' undetected for many generations.


Undesirable Recessive Genes are very difficult to eliminate because they are not expressed or seen until they meet up with an identical partner in a particular mating. Some of the undesirable recessive genes which concern cat breeders are: kinked tails, squints, malocclusions, haemophilia, flat-chested kitten syndrome, cryptorchidism (no testes descended) and monorchidism (only one testicle in the scrotum).


Dominant genes include split foot, and polydactily (abnormal number of toes). Other abnormalities which are thought to be genetic but the exact method of transmission is as yet unclear are: luxating patella (footballers knee), amyloidosis, umbilical hernia, and protruding sternum. Genes for dwarfism, cleft palate, deafness, cardiomyopathy (heart disease) have also been identified in other mammals, eg humans and/or mice.


Agouti and Tabby Genes: All cats have tabby (T) genes in their phenotype but these can only be expressed (or visible) if the Agouti gene is present. The Agouti gene is dominant and causes banding of each individual hair, as found in wild mice, cats and rabbits. When the Agouti gene pairs up with the Tabby gene in the domestic cat, we see many coat patterns: ticked (Abyssinian), mackerel, spotted or classic. The Agouti gene (A) is dominant over the non-agouti (a) gene, ie, the gene for solid colour, as in an Oriental or solid-pointed Siamese. When the Agouti gene is present with the Siamese gene we get a Tabby Point, and where it is paired with the solid-coated gene, we get an Oriental with a tabby pattern.


Mutations: A rare mistake in the process of cell division or, for instance, the effects of radiation, can bring about minute chemical changes which produce a variation in the DNA, or mutation. Several mutations have given rise to the various coat colors and patterns we see in our cats today. Roy Robinson says "long ago ... one of the first mutants to be found in the cat was probably that of self-black" which changed it from the wild, or agouti ticked coat to that of solid black.


In cats, the chocolate gene (b) is known to be a mutation of the black gene (B). It was a natural spontaneous mutation which changed black to chocolate by changing the eumelanin granules to a spheroid shape which refracts the light in a different way, making them appear chocolate. So the Chocolate Point Siamese was a spontaneous mutation and no outcross was used to produce the chocolate color.


Many other feline mutations are obvious, such as the Rex coat, the hairless Sphynx, the ears of the Scottish Fold and the American Curl, the Manx, Japanese Bobtail, and so on.


Pigmentation: Melanin is the substance which causes coloration of the hair shaft. The size and shape of these melanin granules is what determines the color of the cat, and these in turn are controlled by its genes.


The pigment granules in each hair of the cat's coat contains either EumelaninPhaeomelanin which is yellow. Black melanin, or eumelanin granules are thought to be oval in shape and absorb almost all light. Red melanin or phaeomelanin granules are thought to be like elongated footballs in shape and refract light in the red-orange-yellow range. The true red gene, called orange by some geneticists and symbolised by O, converts eumelanin to phaeomelanin, and results in a rich orange-red coat. When the dilution gene is present, the colour appears cream or buff. The red and cream colour in the cat is called a sex-linked gene because it is carried on the X chromosome. To understand its mode of inheritance, we need to know sex and colour of parents. The mating of a red and a black parent can result in the spectacular mixture of shades of red and black seen in the tortoiseshell pattern that generally occurs in female cats. The possible kitten colours from the various combinations of red, tortoiseshell and non-red parents are better understood by the checkerboard colour diagrams as shown in textbooks such as The Book of The Cat. which is black, or


Most reds and creams will show tabby markings to a greater or lesser degree, as the non-agouti gene does not affect phaeomelanin. To achieve a clear-coated red the cat needs to be an agouti (ticked) tabby and highly rufoused. Rufousing (or ruddyness) is an enhancement of ground colour caused by the rufousing polygene(s), as in selectively bred Abys, and some brown tabbies, which show a rich warm apricot ground colour. A lighter brown(b1), which is more reddish than chocolate (b), known as Cinnamon, is a different recessive of the B gene than the one that causes chocolate. It is not to be confused with the sex-linked red/orange gene (O).

Masking genes: Phaeomelanin masks eumelanin, so a red cat can carry black. White is not really a colour, but an absence of colour-the white gene is dominant and masks all other colours completely. The Foreign White is really a cat of quite another colour (usually a Siamese) wearing a white satin nightie! Sometimes Nature gives us a clue by showing a few hairs of the underlying colour on top of a white kitten's head.


Dilution: Yet another mutation is found in the Dense gene (D) which modulates the intensity of Seal and Chocolate Point colour and creates less intense, or more pale coat colours. In the recessive form, the Dense gene (dd) is also known as the 'dilution gene' as it dilutes seal and chocolate to blue and lilac respectively. This diluting gene (d) acts by clumping eumelanin particles, whether elongate or spheroid, into irregular groups causing greatly decreased light absorbtion so that the colour of the fur appears diluted.


Full Colour: The Siamese is a actually a semi-albino, as the Himalayan pattern (cs which is part of the albino series) is caused by a mutation in the tyrosinase gene and is recessive to the gene for full colour C. The cs mutation is a "temperature sensitive mutation". Temperature gradients over the surface of the body control its expression, and the extremities are darker than the body colour due to a greater loss of body heat from these areas.


Temperature sensitive colour pigment: Pointed kittens are born white; colour is not yet developed because the kittens were kept warm inside their mother. Colour begins to develop on the coldest parts of the bodies, ie, the points or extremities; the warmer areas of the body will develop little colour. The darker the points colour, the more insensitive to heat-black is more insensitive to heat than chocolate-hence the difficulty in breeding a Chocolate Point with perfectly even points colour.


Due to space constraints, Tabby patterns, Silvers, and white spotting genes have been briefly discussed and are topics for other articles. The subject of genetics is so broad, fascinating and complex that the aim of this article is only to cover the basics, and perhaps stimulate your interest so you can study more at your leisure.

Applying Genetics to Breeding There are three recognised types of breeding: outcrossing, inbreeding, and line breeding.


Outcrossing means breeding to unrelated stock in order to strengthen one's one lines. The term hybrid vigour says a great deal. The problem with outcrossing is that along with all that vigour can come too many undesirable or unpredictable genes. The number of variables increases.


Inbreeding means back crosses to parents, and crosses to litter mates. It is important to know what is in one's breeding stock, and inbreeding is the easiest and quickest way of learning what is bad, but even more significantly, what is good becomes apparent as well.


Line breeding means breeding to a family of cousins of similar lines that have produced your own stock. Upgrade stock to the best points in each related line, and only line breed to an outstanding individual. The big deal is to get the genes we really want. To do this requires knowledge, patience and goals.


Pure breeding means cats that have registered ancestry, generation after generation standing behind them, and that means they should produce predictable kittens. All things being equal, careful selection is the most important quality, and it is often difficult to be ruthless in weeding out stock which is unsound, unhealthy or has undesirable qualities. Selective breeding is best done by having top quality females in your cattery which can be sent out to the best stud. The alternative is to keep one or two extra special studs in your own cattery, which is often better than taking the risk of infection by sending your queens away.


A good breeder requires patience, a learned or innate ability to see stock objectively, the courage to cull (ie desex and pet out) animals with undesirable characteristics, only keeping sound breeding stock that is in excellent health. No animal with inherited abnormalities should ever be used for breeding. Good feeding is essential; clean conditions and a pleasant environment also contribute to our cats' health and development. It is equally important to limit the number of cats kept, so that adequate care, love and play time can be given to each one.

Breeding is about many things: planning, predictability, revelation, selection, quality, and luck. Quality is beauty, refinement and balance between extremes. In general, extremes of type are to be avoided, as they can be linked to genetic abnormalities. The ideal in breeding is to reduce by careful selection the number of undesirable traits in the genotype so that the most desirable qualities are present in the phenotype. Genetics is about predictability-knowing the pedigree of cats in a breeding program is the foundation. With a measure of good luck, the revelation will be manifest in beautiful kittens. But although beauty is desirable, health, stamina and good temperament are absolutely essential.


In breeding any animal, the primary aim is to improve the breed and produce healthy, distinctive animals that are strong, sound, attractive, capable of reproducing their young without difficulty, and with an unmistakable quality of distinction that stamps the breeder's imprint on litter after litter.


Acknowledgement: My grateful thanks to Kerry Fowler for kindly editing this article, and for her helpful suggestions.



Copyright © Diana F Arnold