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Hints of Genetics

Feline genetics: a combinatorial approach Massimo Picardello

University of Rome “Tor Vergata” Department of Mathematics 00133 Rome, Ital y

The silver colors (smoke, silver).

We now turn to the silver-colored varieties: smoke, silver tabby, shaded silver and chinchilla (and their pheomelanistic equivalents, sometimes called cameo). In these varieties, the base of the hair is depigmented, silver white, without traces of reddish tones. In some silver tabbies, however, the color reaches the root of the hair in the brindle areas. Silver tabbies are brindle cats whose agouti areas are a livid silver hue, with high contrast to brindle. Chinchillas have no traces of branding (in the case of an ideal specimen), and only the tips of the hair are colored (tipping). In shaded, the color on the tips of the hair covers about one third of the length (often less). In the smoke, the silver part is between a third and the initial half of the hair, the rest is colored.

The one-gene theory

In the past, a single main gene,  , was considered responsible for silver colors.

indicated with I (inhibitor), responsible for the braking action on the production of pigment and also for the elimination of the reddish shades typical of eumelanistic colors, especially in the agouti areas of brown tabbies. The mechanism by which a single gene can produce such different color varieties is as follows.

Smokes are non-agouti cats, and the I gene lightens and bruises the base of the coat, giving it a pure silver color, evenly over the whole body. The braking action of the pigmentation produced by this gene can be limited to about a third of the hair, or be greater, reaching the middle of the hair or slightly beyond. All other silver varieties are agouti. Chinchillas and shaded are agouti with a ticked pattern. (Important warning: Chinchilla or shaded silver puppies often display a mackerel or classic [= blotched] phantom pattern, which disappears as they grow. This does not mean that these puppies are not genetically ticked. Ghost bands are common in ticked puppies: ad for example, they are sometimes seen in the Abyssinians. At most, the presence of phantom stripes may be an indication that the cub is heterozygous for the ticked pattern, but this is not necessarily true). Gene I lightens the base of the hair, but without its effect the hair would not have been all pigmented: there would have been various agouti bands, lighter. The action of I brightens the brown tones of these bands and transforms them into silver as well. Since the lightening due to the inhibition at the base combines with the further silvering of the agouti bands above the cut level of the inhibitory action, the percentage of silver hair extends, and reaches about two thirds in the case that the gene I operate in a mild way (shaded silver), and on all the hair except the tip (the last band of color of the ticked hair) in case the action of I is accentuated (chinchilla). That the chinchilla and shaded silver of eumelanistic colors are agouti is also clear from the fact that the skin of their nose is brick red (or deep pink) with the typical external border of brindle. Finally, the silver tabbies are brindle with a classic (blotched) or mackerel / spotted pattern. it is clear that the action of gene I is much more visible on the agouti areas than on the markings of the brindle, and this gives rise to the brindle design with very high contrast.

But just by examining the silver tabbies we see that the genetic explanation of silver colors cannot be based on a single gene. In fact, if this were the case, the brindle markings should be lightened at the base, as in the smoke. But if on the one hand we see silver tabbies of this type, on the other hand we see many others in which the color reaches the root in the brindle areas. Geneticist Roy Robinson, in his well-known book Genetics for Cat Breeders (Cambridge, 1972, 2nd edition), suggests that on brindle areas the action of color filling overpowers silver lightening. But this seems strange, given that on smoke, where the same action occurs on the whole body, the opposite occurs. In any case, it would not be possible to explain why in some silver tabbies there is basic lightening in the brindle areas and in others not.

The two-gene theory

The most modern theories - see the articles by J. Jerome, TICA Trend vol. 13 n. 6 (Dec. 1992 / Jan. 1993) pg. 14 and TICA Yearbook 12 (1991), pg. 218- assert that two distinct genes act on silver, one responsible for the inhibition of the pigmentation at the base of the hair, the other for the elimination of reddish tones (bleaching, bleaching). We will call this second gene the "silver gene" (Sv). The inhibition gene, on the other hand, we will indicate with I, warning the reader on the fact that some recent texts denote with Sh the mild form of the inhibition gene, responsible for the shaded mantle, and with Ch the intense form, responsible for the chinchilla mantle . But there is insufficient evidence that the difference between the two effects is due to two alleles of the inhibiting gene rather than the action of a group of supporting polygens, and indeed the transition from shaded to chinchilla mantle is quite gradual. than net. Therefore we will not refer here to the alleles Sh and Ch, confident that the reader can now easily adapt the conclusions to the context of articles or books that instead refer to them. Therefore, in these notes we do not distinguish genotypically shaded silvers and chinchilla, attributing the difference to secondary polygens.
At this point, the correspondence between the color varieties seen above and their genotypes is clear:

Smoke: aa I- Sv - Shaded silver and chinchilla: A- TaTa I- Sv -

Silver tabby mackerel / spotted:

A- TT I- Sv - if the root of the hair in the brindle areas is silver (silver tabby shell or shaded, depending on whether the silvering is accentuated or reduced); branding is not silver.

Silver tabby classic (blotched):

A- TT ii Sv - if the hair root in the areas of

A- tbtb I- Sv - if the root of the hair in the brindle areas is silver (silver tabby shell or shaded, depending on whether the silvering is accentuated or reduced); branding is not silver.

A- tbtb ii Sv - if the hair root in the areas of

We have indicated the ticked design gene in shaded and chinchilla as homozygous because the dominance of the design genes is only partial. For example, the TaT combination produces a hybrid design, with diffuse ticking partially superimposed on a mackerel design (mainly on the legs and tail, but often a little also on the flanks). If this is the genotype, the uniformity of the tipping is lost. Of course, cats with these characteristics would still be registered as shaded or chinchilla, but their color would be less consistent with the standard.

It should be noted that an ideal silver tabby does not need the action of the inhibition gene I to achieve a perfect contrast between base areas and stripes. In fact, the silver Sv gene lightens and transforms the agouti bands of the hair in the base areas into a pure silver color, which therefore results silvery and livid. Furthermore, the absence of the shading gene I causes the brindle areas to be intensely colored up to the root, increasing the contrast, especially on black silver tabbies, where the elimination of reddish tones makes the black color more intense. is alive.

Other color varieties predicted by the two-gene theory; the golden

If we substituted genotype I- with genotype ii in shaded and chinchilla, we would obtain A- TaTa ii Sv-. Phenotypically, these cats should be silver ticked tabby in the variant

ii, not shaded, that is, with a black band at the beginning of the hair, in contact with the skin. However, it does not appear that these cats were ever produced (see below for further comments). Similarly, if you do the same in the smoke, you get aa ii Sv-, which phenotypically corresponds to solid cats but without traces of rufousing, that is, reddish shades of the hair in eumelanistic colors, or warm tones in pheomelanistic colors. These cats can be obtained by crossing silvers heterozygous for the I gene. If cats of this type are then further crossed with each other, kittens of genotype ii sv sv can be born. From a line of smokes (non-agouti), normal solids are obtained in this case; from a line of shadeds or chinchillas, let's say homozygous agouti AA TaTa, normal ticked tabbies are obtained (like the Abyssinians; however all the shaded and chinchillas are selected against rufousing, that is in favor of polygens of redness of the hair that favor the more bruised, and therefore cannot be expected to obtain, from shaded parents or heterozygous chinchilla Ii Sv sv, ticked tabby puppies with warm reddish tones of the Abyssinians).
But if instead shaded or chinchilla heterozygous for Sv and the inhibitor gene I are crossed, it will be possible to obtain puppies with the I-sv sv genotype. These puppies will be ticked tabby with the base of the hair lighter, but without the bruises produced by the sv gene (but still with not too bright reddish tones, due to the polygenic selection against rufousing). The base shade will be gold instead of red. This variety of color, in the eumelanistic versions, is called shaded golden. The corresponding pheomelanistic varieties are too close in phenotype to red (or cream) ticked tabbies to be classified as a variety in their own right. Similarly, golden tabbies can be obtained by crossing silver tabbies heterozygous for the Sv gene. Here are the genotypes of golden, compared to those of brown tabbies:

Shaded golden and chinchilla golden (golden shell):

A- TaTa I- sv sv

Golden tabby mackerel / spotted: A- TT I- sv sv (note that the root of the hair in the brindle areas is depigmented: golden tabby shell or shaded, depending on whether the depigmentation is accentuated or reduced);

Brown mackerel / spotted tabby: A- TT ii sv sv (the root of the hair in the brindle areas is not depigmented.);

Golden classic (blotched) tabby: A-tbtb I-sv sv (note that the root of the hair in the brindle areas is depigmented: golden tabby shell or shaded, depending on whether the depigmentation is accentuated or reduced);

Brown classic (blotched) tabby: A- tbtb ii sv sv (the root of the hair in the brindle areas is not depigmented.).

Unverified predictions of the two-gene theory

The reader must be warned that this genetic pattern is not entirely satisfactory. In fact, from it we deduce the existence of phenotypes not observed so far. For example, the aa I-sv sv genotype should correspond to a "golden smoke", ie a smoke with a base color of gold rather than silver. But to date, a cat with the color of golden but non-agouti has not been produced. Therefore, it may be assumed that the sv gene is inactivated on non-agouti cats. In this case, the genotypic difference between solid eumelanistic cats with or without traces of rufousing would be due only to the action of polygens.

As already observed, also for the silvers (not golden) there is a dubious phenotype: the one corresponding to the genotype A- TaTa ii Sv-. It should be silver cats but not shaded, that is without depigmentation at the base, and with ticked design. As already observed, under the action of gene I the agouti bands become silver, and for example it is possible to see the effect of multiple alternating silver and black bands in the agouti areas of the black silver tabbies mackerel or blotched not depigmented at the base. But in case the design is ticked, we should have a phenotype with all the fur with alternating silver and black bands, and black at the base (in case the base color is black, of course). It is not known that cats of this type were ever produced. It can perhaps be assumed that the Sv gene is active only if gene I is active, and in fact it modifies and reinforces its action.

The two additional assumptions we have made in this section in order to "rescue" the two-gene theory are equivalent to assuming that the entire Sv / sv allelic series is inactive except if gene I is present. No research has been done yet. experimental enough to confirm this conjecture.

The theory of the golden factor

If this last conjecture is true, in fact the two-gene theory becomes equivalent to another genetic model, which was introduced before the theory based on two independent genes. In this previous model, golden were explained by introducing, however, an additional gene g, whose epistatic action causes golden color instead of silver at the base of the silver coat (as is done to explain the Maltese dilution starting from gene B, with the addition of an epistatic gene d). The dominant allele G has no effect (does not cause a golden color at the base).

Finally, it should be noted that the Sv gene causes a lack of rufousing in eumelanistic colors, but much less in pheomelanistic ones. There are red smokes and red shaded silvers whose tipping is quite hot red (especially in smokes).

The golden theory = brown ticked tabby

Another genetic scheme that was considered consisted in identifying the golden ones with simple cats of ticked tabby color (whose design is ticked all over, without any stripe of brindle; in TICA it is called "agouti tabby"). In fact, because the color of the coat is sensitive to temperature, it is almost always true that ticked tabby cats have a lighter coat base (open the coat of an Abyssinian!), And often the golden and ticked tabby phenotypes are difficult. to distinguish. Faced with the difficulties indicated above, deriving from the two-gene model of silver colors, this simple conjecture that golden is nothing other than ticked tabby is suggestive. In fact, the only phenotypic trait that this theory does not explain is the fact that some ticked tabbies are not very clear at the base while others (the golden ones) are very clear, but perhaps the difference could be attributed to the action of a group. of modifier polygens. Despite the difficulties, however, we will continue the discussion based on the two-gene theory, because it gives the most satisfactory explanation for the existence of two types of silver tabbies: those with black tabby areas up to the root and those with depigmented tabby areas. at the base of the hair. However, we observe that, as regards the most important phenotypic traits, this theory that identifies golden with ticked tabbies provides the same percentages of probability for crosses as the two-gene theory.

The Broadband Gene Theory

Finally, alternative theories have recently been considered in which two genes appear, but with different effects from what has been said above (contributions by H. Lorrimer on the Internet Fancier's List, March-April 1995). In fact, one gene is still the silvering (rufousing elimination) silver gene, but the other would be a gene that causes broadening of the light bands in agouti hair, called wide-band (Wb). However, it should be noted that, due to the large degree of variability in the length of the tipping, it would be more appropriate to speak of a group of broadband branding polygens. In this theory, the problem related to the fact that golden are only agouti (golden smokes are not observed) is obviously solved, but on the other hand the smokes are not explained, which are not only silver (i.e. without rufousing), but also lightened at the base. As these are non-agouti cats, this aspect of their phenotype cannot be due to genes that widen the clear agouti bands.
On the other hand, a variant of this theory can be used to justify a theory advanced in some books: that chinchillas and shaded silver are not necessarily of ticked design. The ticked design, together with the silver inhibiting action, ensures that the color is limited to the tip of the hair (tipping). For g | the other designs this would not be the case, and furthermore the length of the tipping would be far from uniform. But if the agouti bands were dilated by a group of wide-band polygens, and then silvered by the silver gene, then the tipping would be confined to the tip of the hair, and there would be an acceptable degree of uniformity.

The theory [golden = brown ticked tabby + broadband polygens]:
finally an exhaustive genetic model for smoke, silver and golden!
Let us return to the theory that identifies the shaded golden and ticked tabby color varieties. This is

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the most satisfactory theory we have encountered, except for the crucial circumstance which does not explain the partial depigmentation at the base of the hair, a fundamental characteristic of the shaded golden phenotype. On the other hand, there are many cats registered as golden and with varying degrees of depigmentation, some even with almost no depigmentation ... Therefore it must be assumed that shaded golden depigmentation is not the effect of a main gene with net action. (or depigmentation or full color), but the result of the gradual (and sometimes very reduced) action of a set of polygens. But there is already a candidate for this group of polygens: the broadband modifiers introduced in the previous section. Then shaded golds would be nothing more than ticked tabby with broadband base brightening (aside from eye color, which we'll talk about later). If so, everything is fine: the golden must be tabby (no golden smokes!) And partially depigmented at the base (but the amount of depigmentation varies from one specimen to another depending on how much the effects of the individual modifier genes are added together. broadband). Naturally, in this theory the ideal shaded golden results result from crosses of shaded silver heterozygous for the silver and homozygous for the ticked pattern. What if the shaded silver parents are heterozygous also for the ticked drawing? For example, parents might carry the mackerel factor. In this case, if instead of being silver they were brown tabby (no silver), the parents would have brindle lines on the legs, chest and tail, due to the partial dominance of the ticked allele on the mackerel. But as they are shaded ilver, the branding lines will be there, but they will be faint. And in fact, there are some shaded silvers with faint traces of branding (especially from puppies, before the agouti gene reaches maximum effect). However, their golden kittens are not silver, they are brown ticked tabby, and if they carry the mackerel allele the bruises will be clearly visible on the paws, a very undesirable feature in shaded golden, but frequent. To complete the picture, the broadband modifiers transform the other designs (spotted, mackerel and classic) into the corresponding golden varieties (golden spotted tabby, golden mackerel tabby and golden classic tabby respectively), in which the agouti areas are partially depigmented at the base and have warm apricot tones.

Now we finally have a satisfying golden theory. However, in order to be compatible with the silver one, it is necessary to review and correct the latter. We can no longer hypothesize two genes for silver, one for depigmentation and the other for silvering. Instead we have to go back to the one-gene theory, an inhibitory gene that we will continue to call I and that simultaneously causes silvering and depigmentation, as in smokes, for example. It should be noted that the golden genotype is not due to gene I (indeed the golden ones are heterozygous ii): the depigmentation at the base of the golden hair is instead due to the elongation of the first agouti band due to the action of the broadband modifier genes. We do not introduce a distinct main gene that causes depigmentation, otherwise, as we have seen, the theory would predict the non-existent golden smokes (golden non agouti, depigmented at the base of the hair). On the other hand, it would seem at first glance that this loses the brilliant explanation that the two-gene theory gave regarding the fact that some silver tabbies are depigmented at the root of the hair in the brindle areas and others are not. But this genetic trait can be explained in some other way. The gradual level of depigmentation at the base of the hair in the brindle areas may be caused by modifier genes similar (or possibly identical) to broadband modifiers, whose action is limited to tabby cats. With a slight conceptual abuse we can imagine that these additional modifiers belong to the same group as the broadband polygens (in reality this is not the case, because the latter act on the agouti base areas while the former act on the brindle areas: but in reality they meet many golden in which broadband depigmentation also occurs on the areas of tabby marks ...).

In summary, compared to the two-gene theory our new genetic model retains the inhibitor gene I (which is now considered responsible for both depigmentation and silvering, as in the one-gene theory), and replaces the second gene with a group of polygens. , which we will denote by Wb (for "wide-band"). What is particularly satisfying is the fact that the gradual action of broadband modifiers can explain the difference between the shaded silver and chinchilla phenotypes. Unfortunately, however, we are assuming that these are polygens, which therefore are not subject to the simple and direct rules of Mendelian genetics for the main genes. In order to analyze the mathematics of this new model it is necessary to resort to statistics, the results of which are too fine to be clearly distinguished in the phenotypes. Therefore it is not easy to present the results by means of cross tables.

Silver and golden

It should be noted that the deepening of the single gene theory presented in the previous section, as well as the variant introduced previously, excludes the possibility of cats at the same time silver and golden: the silver phenotype is a consequence of the inhibitory gene, which the golden they do not have. Both shaded silver and golden have broadband type color suppression or lightening genes, or polygenic modifiers of the same type. In the latest version of the theory that we presented immediately above, the golden pattern is caused by the suppression action of this type of genes superimposed on hairs with already banded coloring due to the Agouti gene: thus, if the tabby design is the ticked, you get the perfect golden pattern, uniform throughout, but if the design is mackerel, spotted or (even worse) blotched, then more or less large areas of tabby marks become evident with full color, not very lightened, only orange in color in the band at the base. These patterns could be classified as golden tabby (mackerel, spotted or blotched, respectively). Since in such cases the areas where the coat has a broad golden base are less broad, these phenotypes are more difficult to determine with certainty, particularly in kittens, especially if they are phantom patterns in young cats due to heterozygous tabby genotypes with a only ticked allele, and some origins books (for example that of TICA) do not recognize them.

Finally, it should be noted that the action of broadband suppression genes creates long agouti bands of pheomelanistic color, which are difficult, if not impossible, to recognize on cats of already pheomelanistic base color. If the lightening band is not long this is easier, just like in the case of the tortie ticked tabby pattern.

Eye color of shaded and golden: an example of persistence?

In relation to the similarity between golden and ticked tabbies, it should be remembered that in ticked tabbies the color of the eyes is gold or copper (in the case of Persians) or also hazel or yellow or yellow-green or green (in the Abyssinians). In the shaded silver or chinchilla of eumelanistic colors the eye color is not gold or copper, but emerald or blue-green. Instead, in the shaded silver or chinchilla of pheomelanistic colors, the eye color is copper. With appropriate crossings and consequent selection, it has been possible to introduce the gold or copper color in the eumelanistic shaded silver: the corresponding standard, recognized by some feline associations but not all, is called black shaded (in England, pewter). So the difference between an eumelanistic golden and the corresponding ticked tabby, although it may be barely visible in the color of the hair, is still evident in the color of the eyes, emerald in the first but not in the second. In silver tabbies, on the other hand, the eyes are green or yellow-green or gold, with a preference for green. In the shaded and in the pheomelanistic silver tabbies the color of the eyes is copper, and in the corresponding tortoiseshell varieties it can be copper or emerald (shaded) or green (silver tabby), with preference for copper.

The existence of black shaded with gold eyes reveals that the emerald color of the eyes is not caused by the silver gene. It is interesting to observe that, although the color of the eyes, emerald or copper, is largely determined by a main gene, it is not common to meet eumelanistic shaded silvers with copper eyes: the emerald color occurs more frequently. So, although the color of the eyes and the hair depend on different genes, somehow there is a persistent link between the shaded silver coat and the emerald color of the eyes. This link is perhaps due to the fact that the main genes that cause this eye coloring are located on the same chromosome as the genes that cause the shaded silver color of the coat, and therefore the genetic transmission of one and the other occurs together. A situation of this type, which is called persistence, modifies the probabilities of concomitant genetic transmission of the two genes, which are very different from what they would be for two independent genes. To break the persistence, in fact, it is necessary that in the close contact of the two homologous chromosomes during the pairing that precedes meiosis, a genetic recombination takes place, that is a molecular exchange between the two chromosomes.

The action of the I and Sv genes is epistatic on that of the solid color, eumelanistic and pheomelanistic genes, but not on that of the white (W) and piebald spot (S) genes. Obviously, the W- B- A- TaTa I- Sv- genotype corresponds to a phenotype in which white masks the shaded silver color on the coat, but the eye color can be either blue (due to the depigmentation action that the gene W can exert on eye color) or emerald.

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