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The genetics of Maine Coon colors

Autosomal chromosomes are those chromosomes found in an organism's body that control cell function and physical appearance (as opposed to the chromosomes found in the female's egg or the male's sperm, which pass genetic information down from parent to offspring). A cat has 18 pairs of autosomal chromosomes and one pair of sex chromosomes. That makes 38 total, and a cat gets half of those chromosomes from the mother and half from the father.

Fertilizable eggs in the female and sperm in the males (both referred to as gametes) are produced during a process known as meiosis. At its conclusion, both gametes contain a total of only 19 chromosomes. In other words, a gamete will possess one half of the full complement of chromosomes, one from each homologous pair of chromosomes. That way, when the egg and sperm merge (right), the offsping has the requisite number of 38 chromosomes.

Tabby markings

Tabby patterns consist of black pigment (eumelanin) against a yellowish ground color (phaeomelanin). The agouti gene controls the deposition of color by producing a molecule called the agouti protein. As a hair grows, eumelanin is deposited into the hair. As the amount of agouti protein increases, eumelanin production is inhibited, and there is a shift to production of phaeomelanin that is then deposited into the hair, creating a hair that is black at the tip and yellow at the base This type of coloration is referred to as ticked, agouti coloration.

In cats, a second system of pigmentation is responsible for the dark stripes interspersed throughout the agouti coat, caused by a marked reduction in the amount of agouti protein receptors (or agouti protein itself) in certain areas of the skin, eliminating the eumelanin to phaeomelanin shift. Hairs within these regions incorporate the eumelanin pigment from base to top, creating the overall impression of one dark-colored pattern imposed another pattern, the agouti coloration (right).

The allele on the location of the agouti gene that produces agouti coloring is symbolized by the letter A and is dominant (hence the use of the capital letter) over the the non-agouti form symbolized by a. All tabbies are either AA or Aa, identical in appearance but different in genetic constitution. The outward appearance is referred to as the phenotype of the cat and the genetic constitution (the alleles present) as the genotype of the cat.

Remember, a second system of pigmentation is responsible for the dark stripes on a cat. There is a second gene at play, responsible for the pattern, which interacts with the agouti gene. The alleles present at this second gene location can be either MC or mc. The allele responsible for the mackerel pattern is designated by the letters Mc, and the allele for the classic pattern by the letters mc. The use of the capital letter "M" to designate the allele responsible for the mackerel pattern reflects the fact that the mackerel-pattern allele MC is dominent over the classic-pattern allele mc. Since each cat has a pair of chromosomes (one inherited from its mother and one from its father), cats possess one of three possible variations in their genetic code (or genotypes) affecting pattern: McMc, Mcmc or mcmc.

Can we predict the pattern of the offspring if we know the pattern of the parents? Statistically speaking, if the father's genotype is McMc (a homozygous mackerel tabby, each of whose homologous chromosomes contains the Mc allele) and the mother is mcmc (a classic tabby, since she possesses both recessive alleles), then all of the offspring will be MCmc. That is because the only allele that the father can contribute is an Mc, and the only allele the mother can contribute is an mc. All the offspring will appear as mackerel tabbies but be heterozygous for the mackerel pattern.

 

To illustrate this and other examples that follow, charts can help make the statistical possibilities easier to visualize. We will use certain conventions. The title of the chart lists the breeding, showing the mother's genotype first (in this first example, mcmc) followed by the father's McMc genotype. The possible contributions from the father are represented as column headings and the possible contributions from the mother are the row headings. The possible offspring genotypes are inside the chart.

In thi first example, we have already noted that the only allele that the father can contribute is an Mc, and the only allele the mother can contribute is an mc, so all the offspring will be mcMc (listing the mother's mc contribution first in the offspring's genotype).

Now let's pair, say, one of these Mcmc offspring (say a female, for purposes of drawing the chart) with a classic tabby male (which must be mcmc). Those offspring will all receive an mc allele from the father. The mother's contribution is more complicated. Half the offspring will inherit the Mc allele from the mother, and the other half, the mc. That means half the offspring will be mcMC (heterozygous mackerel tabbies, capable of passing either the mackerel or classic tabby allele to their own offspring) and the other half, mcmc (classic tabbies).

In conclusion, if one of the parents has a mackerel pattern, at least some or perhaps all of the offspring will exhibit the mackerel pattern, since the mackerel allele is dominent. In addition, no mackerel tabby offspring can result from the mating of two classic tabbies. In more technical terms, if the phenotype of both parents is classic tabby, their genotypes are both mcmc, and each can pass only the mc allele to their offspring to create more mcmc (classic) tabbies.

Brown or black or even blue?

To explain the genotypes of brown, black and gray (cat fanciers call this color "blue") colored cats, we need to understand the influences of three different genes.

As we discussed earlier, the agouti gene controls color deposition. The agouti allele A at this gene location causes individual hairs to have bands of alternating light and heavy pigmentation (or coloring), creating the yellow-beige appearance of the tabby's ground color. Eumelanin (black melanin) is the pigment responsible for the darker colored bands on the hair shafts. The mutant non-agouti a allele known as non-agouti produces a defective agouti protein that does not have the strong inhibiting effect on eumelanin production as the normal protein, causing eumelanin-containing granules to be deposited throughout the hair shaft during its entire growth. No stripes will be present in the coat of a cat with a genotype of aa.

A second gene, the dilute gene (located at a separate locus on the chromosome from the agoiti and brown loci) produces a factor essential for even distribution of pigment granules throughout the hair. The D allele is responsible for dense coloration. In its mutant recessive d form, the dilute gene produces enlarged pigment granules that deposit unevenly in the hair shaft. Segments of the hair may be only sparsely pigmented or lack pigment altogether. This "impairment" causes the coat to appear as a lighter shade, as more light is allowed to pass through the hair. The d allele is recessive, so in cats homozygous for this allele (dd), black appears gray (referred to as blue by cat breeders). A red cat that has a genotype of dd is cream.

Now, let's put together what we have learned about these first two genes. What are all the possible variants of alleles at the two loci, one for the agouti gene and the other, black pigment gene? The genotype of a tabby homozygous for both the agouti gene and the dense coloration gene on both bomologous chromosomes is represented as AADD. Opposite in genotype is a solid blue represented by aadd (non-agouti and dilute on both homologous chromosomes). Since the A and D alleles exhibit dominance, an AADd, AaDd or AaDD will all appear to be brown tabbies (the phenotype). The phenotype of AAdd, Aadd or Aadd is blue tabbies, and aaDD, aaDd and aadd will be solid blue.

Here's how the numbers work out... A parent passes one of its A gene alleles and one of its D gene alleles to each of its offspring. A parent that is homozygous in both the A and D loci is AADD, and will produce all AD gametes. On the other hand, a parent that is heterozygous at both loci (AaDd) can produce several variants among its germ cells: AD, Ad, aD or ad. If you breed a brown tabby heterozygous at both the A and D loci (AaDd) with a brown tabby that is homozygous at both loci (AADD), all the offspring will still be brown tabbies:

 

If you breed two brown tabbies that are heterozygous at these two loci, here's how the numbers work out statistically:

A third gene is at work to complete our understanding. The site on a chromosome commonly referred to as the brown gene controls the shape of the eumelanin pigment granules that are deposited. As the shape changes from its normal round shape to oval, the normal black appearance created by the normally round granules is changed into different intensities of brown. The B allele (black) or b (chocolate) do not appear to modify the phaeomelanin pigment-containing granules or, if they do, they seem to produce a brighter shade of color.

Here are some exemplary genotypes that combine the influences of all three genes: agouti, brown, and dilute among Maine Coons.

Solid black = aaB-D- (a densely colored black cat without stripes)

Solid blue = aaB-dd (a dilute black cat without stripes)

Brown classic tabby = A-B-D-mcmc (a densely colored, genetically black cat with stripes and classic markings whose rich brown ticking in the hairs that make up the stripes has been produced by polygenic rufousing not fully understood)

As you can see,, the exhibition brown classic tabby is the product of many influences, and today's look (or phenotype) is the outcome of selectively breeding cats that possess the desired color nuances, not all of which are fully understood.

 

Red cats

Among the 19 pairs of chromosomes, some pairs are larger than others and have different shapes, but the chromosomes within each pair are generally identical in size and shape. That is, with one important exception. One pair in males is unequally sized, one chromosome in the pair being larger than the other "matching" chromosome, whereas that same pair in females matches in size and shape. The chromosomes in this one pair are known as the sex chromosomes. Whereas females have two X chromosomes (of equal size), males have one X chromosomeone and one (smaller) Y chromosome. The Y chromosome contains the gene that triggers embryonic development as a male. For sex-linked traits, the genes that control those traits are located specifically on the X or on the Y chromosome, so those traits are linked to the sex of the cat.

The gene responsible for the marmalade colored cat is referred to as the orange gene, and this gene is sex-linked, carried on the X chromosome. The O allele, converts all eumelanistic (black) pigment to an alternative pigment, phaeomelanin (or red melanin).

Male cats have only one X chromosome, inheriting their Y chromosome from their father. So, male offspring can only inherit the O allele on the X chromosome from their mother and will be red in color.

An interesting aspect of the genetics of the orange mutation is the the Oo female is the tortie, a striking mosaic of orange and black. You can only get a red female offspring (OO) if the father is red (OY), where the "Y" represents the absence of an orange allele on the Y chromosome) and the mother is either Oo (tortie, and the female offspring inherits the O, not the o) or OO (red).

To predict the results of matings with sex-linked genes, it is necessary to know the gender of the cat(s) contributing the O allele. Males have two sex chromosomes, an X and a Y. Theoretically, half of a male's gametes will carry an X chromosome and the other half will carry a Y chromosome. With sex-linked traits, the genes controlling that trait are located on one of the sex chromosomes, so the male can only transmit an allele for that trait in 50% of gametes. Since the orange gene is located on the X chromosome, those gametes with the X chromosome will carry either the O or o allele. The gametes with Y chromosomes will not carry an orange gene at all. So, male cats have only one O (red) or o (non-red) allele.

 
Now the combinations get a little tricky, so we'll start with an easy example. Consider the mating of a red female (OO) to a red male (OY). Here's a red female (who can only contribute O gametes) crossed with a red male:
A tortie female (Oo) crossed with a red male (OY) can also produce red female offspring:
How about a red female (OO) and brown male (oY)?
Now, a brown female and red male:

Adding the effect of the D gene in the genotype of a cat, the cream cat is the dilute variation of the red cat, and the blue cream cat is the dilute variation of the tortoiseshell. Here are some interesting phenotypes and their corresponding genotypes:

Cream male = ddOY

Cream female = ddOO

Blue cream female - aaddOo

A red or cream tabby's stripes are shown against a lighter orange or yellow background. Curiously, the red or cream tabby can exist in two forms: as an agouti or as a non-agouti. Since the non-agouti allele produces an agouti protein that does not inhibit black pigment but does inhibit the orange pigment, the tabby pattern phenotype is seen in reds and creams no matter what the genotype for the agouti A gene. This phenomenon is called masking, where one gene (the O gene, in this case) masks the presence of other alleles (here, the agouiti vs. non-agouti allele on the A gene).Silver tabbies

The dominant allele I on the inhibitor gene suppresses pigment fed into growing hair shafts, with the result being white hairs with colored tips. Phaiomeaninprigment is supressed to a greater degree than eumelanin pigment, preventing the eumelanin to phaeomelanin shift. This gene has wide variation in expression, ranging from a barely perceptible white band at the base of the hairs next to the skin to an almost completely white cat that has pigment ony on the extreme hair tips. This phenomemon is known as incomplete penetrance, in which a trait fails to be manifested even though the genotype indicates that it should, and the cat produces offspring expected from a parent possessing the genotype.

The silver tabby manifests a fairly low level of expression of the I gene. A silver tabby's genotype is A-I-, where the "-" stands for either variant A or a in the agouti A gene and I or i in the inhibitor I gene, depending on whether the cat is heterozygous or homozygous for each trait. Occasional break-throuogh production of phaeomelanin pigment can occur in silver cats, resulting in what we call tarnishing. Where you would expect the color to be silver, the color is golden to brown where some phaeomelanin is deposited. This phenomenon appears in cats heterozygous for the I allele (Ii). Look very closely at the face of the silver kitten above, and you can see very small amounts of yellow hairs, especially under the eyes.

The smoke color is formed by the combination of the inhibitor I gene with the homozygous non-agouti gonotype (aaI-). A white undercoat is evident but each hair contains appreciably more pigment due to the lack of the additive inhibitory properties of the agouti factor.

White spotted cats

White spotted, or piebald, cats are common. The spotting can occur with any color cat. The amount of white may be limited to small tufts of white hair on the chest or belly to an almost completely white cat with pigment areas confined to the tail and perhaps small spots on the head or body. As the belly becomes largely white, the neck and chin and front paws become increasingly involved. As the amount of white increases further, the cat's sides, head and hind paws become spotted with white.

Piebald spotting is the result of the semi-dominant S alelle on the spotted gene. The genotype of any white spotted cat is assumed to be either Ss or SS. Because of the extreme variability in expression in this trait, we can only assume that some medium- and all high-grade spotted cats have homozygous SS genotypes, and low- and medium-grade spotted cats have heterozygous genotypes Ss. It is assumed, but not proven, that the variabiity in expression is due to polygenes (the existence of more than one gene influencing the others).

As white spotting is a dominant character, at least one parent must possess the spotted phenotype (appearance) in order to get kittens with white.

This S gene also affects how the genotype Oo in tortoiseshell female cats is expressed. Without any white spotting, the orange ) and black (o) colored fur is intermingled to create a blended mosaic look. The spread of both O and o cells is believed to occur at equal speed. The presence of white in the phenotype is seen to cause the orange and black to form patches of color. Generally speaking, the greater the amount of white, the larger the patches.

White cats

The completely white cat is due to the dominant gene W. This gene is fully dominant, so the presence of one gene is sufficient to create a solid white cat. By looking at the cat, it is imposssible to tell what other genes are present in the genotype at the A, D, O or I genes. The W gene is said to be epistatic, masking the effect of all other color genes. So, technically, white is not a "color" in the same sense as the black or blue color in cats; the W allele masks the underlying color of the cat.

Breeding a white cat and noting the phenotype of the offspring can give you clues as to the complete color genotype of the parent cat. Breeders have noted that kittens of a white cat may possess a small spot of color on the top of the head, giving another clue, but the kitten's few pigment-producing cells rarely persist into adulthood.

White cats can have either blue, non-blue (gold or green), or odd eyes (one blue eye, one non-blue eye). Deafness is more likely to occur with blue-eyed animals, but the association is not consistent. The deafness, like the eye color, can be bilateral (affecting both ears) or unilateral (affecting only one ear). Cats with white coats are also more susceptible to skin cancer, and care should be taken to keep them out of strong sunlight.

If you would like to learn more, here are two sources: Robinson's Benetics for Cat Breeders & Veterinarians and Genetics for Cat Breeders by Roy Robinson.

 
 
 

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