.
How to Paint a Cat
Introduction The purpose of this introduction is to provide a basic understanding of how color inheritance works in cats. Some years ago I wrote a booklet for children called The ABCs of Color in Cats. Here we have a more mature description of color inheritance covering major cat genes, masking, genotypes of solid color cats, crossing of homozygotes and heterozygotes, white spotting, reds and torties, color at the points, tipped colors, tabbies, test-mating, and eye color. Mendel - father of genetics. The science of genetics began with a man named Gregor Mendel, an Augustinian monk who lived in Austria. Mendel was a self-taught naturalist who did substitute teaching in high school science classes. After failing to qualify as a regular teacher Mendel decided to enter a monastery. While there he was sent to the University of Vienna in 1851 for two years of study in math and science. Prior to Mendel, although there was considerable lore about animal breeding, no one had applied statistics to biology in a systematic way. Mendel’s work was published in 1866, but went virtually unnoticed until about 1900. Mendel began his experiments with strains of true-breeding peas. Mendel developed the basic laws of genetics, applicable to both plants and animals, by experimenting with 34 true-breeding strains of peas which varied in such traits as seed form, color of contents, color of seed coat, color of unripe pods, shape of pods, length of stem, and position of flowers.
Dominant vs. Recessive. By crossing true-breeding strains of peas Mendel discovered, for example, that when he crossed true-breeding round peas with true-breeding wrinkled peas, the seeds from such a cross were all round. He called this trait, the dominant trait, and the one that did not appear, recessive. This generation of peas, the first generation of offspring, he referred to as the F1 generation. Next Mendel found that when he crossed the peas from that F1 generation with one another they were no longer true-breeding, but produced one/fourth wrinkled peas and 3/4 round. When he repeated this experiment with a number of other traits he found the same interesting ratio; 3/4 showed the dominant trait and 1/4 the recessive. We see this same result in animal genetics. If there is a mating between two animals which are heterozygous for a trait their offspring will show both the dominant and the recessive trait in a 3:1 ratio. An exact 3 to 1 ratio may not be shown in every litter, but emerges when the breedings are repeated a number of times. Capital letters are used to represent the dominant gene; lower case letters to represent the recessive gene.
Homozygous vs. Heterozygous. Mendel had discovered that what you see is not necessarily what you get, not all round peas breed true. He used the term homozygous (meaning one zygote) to apply to the pure-breeding peas and the term heterozygous (different zygotes) to apply to the peas which could produce both the dominant and the recessive traits.
Dense vs. Dilute. We apply these terms today in the breeding of cats to explain, for example, that a homozygous, or pure-breeding cat which is black (BBDD), can only produce kittens of a dense color (denoted D), while a heterozygous black cat (B-Dd) can produce kittens of both dense colors (e.g. red, black and chocolate), and also the dilute colors (denoted dd), of cream, blue and lilac, depending also, of course, on the genotype of the cat to which it is bred.
Phenotype and Genotype. Mendel needed terms to distinguish between how an individual looks, which he called the phenotype, and how it behaves genetically, its genotype. For example, 2 black cats may not be discernibly different as to looks, or phenotype. These two cats, however, may differ as to genotype; one of them may have a recessive gene (c) for the Himalayan coat pattern, and could produce Himalayan kittens when bred to a Himalayan (cc). Similarly, if one is working with exotics, which have short hair (L-) on a Persian body, one might breed together two exotics which were heterozygous for the gene for hair length (Ll), and thus obtain offspring in the familiar 3:1 ratio, three with short hair, (dominant), and one with long hair (ll), which is recessive.
Progeny-testing. You and I know that we can’t tell whether that black Persian has a Himalayan gene just by looking, whether it is homozygous or heterozygous for that gene for solid coat. Mendel wanted to learn how to tell the homozygous from heterozygous peas. By experimenting, he learned that he could distinguish between the heterozygous and homozygous peas by crossing them to peas that were pure-breeding (homozygous) for the recessive trait. In the cat fancy, if we have bred two cats together which we know are carrying the Himalayan gene we are uncertain as to the genotype of their solid color offspring. Those which look like Persians may carry both the gene for full coat color (C) and the gene for Himalayan coat pattern (c) , and we need to test-mate them to a Himalayan (cc), in order to determine whether they carry the recessive Himalayan gene. This is a form of progeny-testing in which we determine by breeding what the genetic characteristics of the parents may be.
Undesirable recessives. Test-mating may also be used to determine whether a cat has a gene for an undesirable recessive. It is sometimes estimated that, on the average, most individuals, people and cats alike, harbor about five undesirable recessives. (Since we usually do not mate with close relatives, we just don’t know what recessives we carry.) If you have a litter in which one of the kittens has a cleft palate, or a tail kink, cryptorchidism (undescended testicle(s), congenital cardiomyopathy, progressive retinal atrophy (blindness) or an umbilical hernia, it is quite likely that both parents carry the recessive genes for that trait. These defects are commonly agreed to be due to recessive genes, which must be carried by both parents in order to be expressed.
Eliminating recessives. As a breeder, you have some difficult decisions to make as to how radically you are going to select against undesirable recessives, i.e. whether to blithely ignore them as a fluke, give up cat-breeding; whether to neuter the male, spay and neuter both parents, or both parents and all their kittens. If you continue to use the male as a stud he will pass that gene on to half his kittens, even if only one of your females shares that recessive. You might keep him intact to use for occasional test-mating to determine which females also share the trait. The decision you make may depend upon the seriousness of the trait, at what age it is manifest, whether it is lethal, what other males you have, the value of his other characteristics for your breeding program, your goals as a breeder, etc. Because the littermates of affected kittens have a 2/3 chance of also carrying the gene, you might wish to test-mate the females in the litter to decide which should be used for further breeding or even to test-mate the males to find a replacement for the sire which does not carry the gene. Your chances of securing a male which does not share the undesirable recessive will be much better if you choose a male kitten from a queen whose kittens, sired by the same male, have not shown this trait. It is reasonable to assume then that they have a 50% chance of receiving it from their sire, less than the probability of receiving it if both parents carried the gene.
Meiosis. Before one can understand color charts it is necessary to remember that when cells divide to form sperm or ova the process, meiosis, is different from ordinary cell division. Although all the other cells in the body contain pairs of chromosomes, when ova and sperm are formed all these pairs of chromosomes are separated and each sperm or ovum receives only one of the pairs of chromosomes at each site. In that way, when the ovum and sperm join at fertilization there are once again paired chromosomes for the new individual.
Alleles. Each gene has a particular location on the chromosome for each individual of a species. Often there will be two or more possible genes for a characteristic. These differing genes for a particular characteristic, for example, long hair (l) vs. short hair (L) are called alleles. They are at the same location on the chromosome, and therefore are alternatives. The allele which is dominant is designated with a capital letter, and the recessive allele is designated with a lower case letter. For a particular cat a genetic description of major color genes might look like this: Aa - The cat has one dominant agouti gene (A) which makes stripes, and one (a) for solid color Bb - The cat has one dominant gene for black (B), and one for chocolate (b) cc - This cat has two copies of the recessive gene (c) for the Himalayan coat pattern Dd - This cat has one dominant gene for dense coloration (D), black (vs. dilute-blue) Ll - This cat has the dominant gene for short hair (L) - the phenotype? Seal lynx point exotic
Independent Genes. Some genes tend to be inherited together. When trying to determine the probability of a cat receiving a particular gene, or genes, it is important to know whether we are considering genes which are inherited independently or not. If the two genes are inherited independently the probability of both of them occurring is equal to the product of their individual probabilities. Thus when 2 black cats which are heterozygotic for dilute (Dd) are crossed the probability is for one in four to be dilute. If these cats are heterozygotic for long hair (Ll) the probability is that one in four will have long hair. However, if we seek the probability of a blue long hair from this breeding the probability is only 1 in 16, the product of the 2 separate probabilities, 1/4 x 1/4. The importance of this finding has led to its being termed the Multiplicative Law - that the probability of 2 independent events both occurring is equal to the product of their individual probabilities.
Gene linkage. Not all traits are inherited independently in cats. For example, the Siamese coat pattern is closely linked with blue eyes, and whenever this pattern is seen, whether it be in a Siamese, Balinese, Birman, Himalayan, or Tonkinese, the eyes will be some shade of blue. This may be because the genes for blue eye color are located close to the genes for the Siamese coat pattern, or due to a chemical effect - an enzyme which affects both coat pigmentation and pigmentation of the iris in the eye. It is known that the light-colored body coat in color points is caused by an enzyme which interferes with pigment development on the areas of the cat’s body which are slightly warmer. This is an example of an interaction between the genetic constitution and the environment.
Penetrance. Some genes vary in their ability to be expressed, or penetrance. Polydactyly, the gene symbolized (Pd), which produces extra digits in cats, may produce varying numbers of additional digits. I have seen up to 11 to 13 toes, way beyond the normal five on the front and four on the back. Penetrance may also be affected by the sex of a cat. For example, a female cat may receive two genes for cryptorchidism, but cannot manifest the disorder, since she does not have testicles. One might thus select against using the female littermates of a male which is cryptorchid, since there is a more than 50% chance that they will pass the gene to their offspring. This gene also varies in its penetrance in other ways; some cats inheriting two copies of the gene may be cryptorchid, some monorchid, and some may appear superficially normal, but have an underlying structural weakness and be capable of passing the gene to their offspring.
Modifying polygenes. In addition to the major genes which affect color there are probably some modifier genes which affect color. Examples of such genes might include, for example, (1) the genes which affect the placement of white spots in piebald white spotting, (2) some which are responsible for lightening the blue color on blue cats, and (3) some for the “rufusing” or intensifying the red color between the stripes on red cats. It is generally agreed that these are due to polygenes, or groups of genes acting together to produce discernible differences in shade of color among cats which are the same with respect to major genes.
Masking - epistasis. In cats there is a dominant white gene, symbolized (W), which is an example of epistasis, or masking, as white is inherited, in addition to whatever other color genes or patterns the cat may be carrying. The dominant white gene is often accompanied by deafness, especially when the animal is homozygous for that gene. Deaf cats should always be kept inside to protect them from hazards. If bred, the deaf mother cat may need to be kept particularly close to her kittens, as she is unable to hear their cries. The cat which is a white heterozygote (Ww) will often be born with a color spot on the head of whatever color is being masked, giving a clue as to what color genes are being masked. This color spot usually disappears during the first year. Those white cats which are masking two copies of the Himalayan gene will have blue eyes, no color cap and no deafness. When bred to a Himalayan the probability is for 50% blue-eyed whites and 50% Himalayans, with no deafness, since the blue eyes are based upon the Himalayan gene.
Sex Chromosomes. Most chromosomes occur in pairs which appear similar in form, but not the X and Y sex chromosomes. The Y chromosome contains only the information necessary for male sexual development. Females of a species receive one X chromosome from both ovum and sperm, one from each parent, while males become male by virtue of receiving a Y chromosome from the sperm as well as an X chromosome from the ovum. Normal males thus have an X and a Y chromosome throughout their body cells, and when sperm are formed 50% of these carry the Y chromosome and 50% carry the X chromosome. Thus, the male bears the major responsibility for determining the sex of the offspring. The fact that the Y chromosome is an ’abbreviated’ chromosome, carrying only the genes necessary for sexual development, has implications for what is called sex-linked inheritance, but that is a story for another day.
 

























 

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