Once again about the dominant gene for blue eyes
in the cattery Ermine Trace
III. What are the dominant genes for blue eyes?
Having begun to better understand the nature of DBE, we found that mutations that affect melanogenesis [melanin formation] to produce a blue-eyed or odd-eyed phenotype coupled with a small amount of white fur, are far from uncommon. There are not merely one or two known mutations of this type, there are dozens. They can be located on different chromosomes, in different genes, among which the best known are PAX3, MITF, EDNRB and others. Humans have similar mutations, not to mention mice, ferrets, dogs, horses, etc. However, it was the human mutations that became most famous thanks to the Dutch scientist, ophthalmologist and geneticist Petrus Johannes Waardenburg, who studied and described their manifestation in 1948-1951. In his honour, the corresponding phenotypic traits in humans were given a standard medical name according to established tradition, and bore a pronounced negative connotation: "Waardenburg syndrome".
It must be said that many, if not all, mutations that become breed- or colour-forming in domestic animals instantly fall into the category of "syndromes" and "hereditary diseases" in humans. The same is true of albinism, and indeed any deviation from normal skin pigmentation in humans; it is automatically interpreted as a disease. Mutations in the ASIP genes are diseases in humans because they cause severe metabolic disorders, e.g. "yellow obesity," but in cats they only cause changes in the banding of the hairs. In humans, TYR mutations cause oculocutaneous albinism and a predisposition to skin cancer, but in cats they just cause the colour-point pattern. KIT mutations in humans cause the piebaldism skin disorder and contribute to oncogenesis [the process through which healthy cells become transformed into cancer cells], but in cats it only causes white spotting. And so on.
At present, we cannot say for sure which of the many mutations affecting melanogenesis we are working with. The only thing we can confidently say is that we have only one such mutation on our gene pool, and this is the one that we first discovered in a particular British Shorthair cat, the silver chinchilla named Ermine Trace Nadeya. Thus, Ermine Trace Nadeya is the only founder of our small lineage within the British chinchilla population. All our blue-eyed and odd-eyed cats are her direct descendants, and they have no other unrelated ancestors known to be DBE latents of possible latents. This is a fundamentally important point on which the success of genetic research depends. In order to detect a new mutation, it is necessary to isolate "suspicious" DNA regions in two groups of cats: known carriers of the mutation and "wild-type" cats, in which the desired mutation is absolutely absent. The selected DNA areas are compared with each other and reveal the general differences between two groups. If the same change in the nucleotide sequence is present in the DNA of all cats in the first group, but it is not present in the "wild-type" control group, it can be assumed that this change is the mutation we are interested in. If all mutation-bearers in the first group turn out to have different mutations with a similar phenotype, then all the work of finding common differences becomes meaningless: you cannot find something that is not in the samples provided.
Now, in 2021, the first of Nadeya’s great-grandchildren (F3) were born, and her great-great-grandchildren (F4) are expected to be born soon. This is just the very beginning of our work. Unlike our DBEs, some DBEs have been studied for much longer than just 5 years. However, the genes in which the corresponding mutations are located are still unknown even to the breeders who are working with the descendants of the very first blue-eyed cats, the ancestors of the Altai breed and the Topaz breed. In addition, many do not limit themselves to working with one DBE foundation cat and its descendants, but use several blue-eyed animals of different origins in their breeding program at the same time. As a result, no breeder today can be completely sure that the DBE s/he is working with is the same as the DBE used by his predecessors. As a result, s/he may well get a cat which is not a homozygote, but which possesses two different non-allelic DBEs at the same time without knowing how the genes interact with each other. The same breeders who work with DBE, whites and bicolours (harlequins, van) at the same time cannot be at all sure that their kittens have the DBE gene, and not one of the alleles of the W series. (white and white spotting) which are well-known and where blue eyes surprise no-one.
Nevertheless, through 20 years of breeding, breeders have accumulated a huge amount of practical material, although it is still not in demand by scientists. None of the numerous attempts by breeders to initiate genetic research and identify the types of DBE at work have been successful. The only exception is the studies of DBE Ojos Azules, which were carried out by the American geneticist Solveig M.V. Pflueger in the late 1990s - early 2000s, studies that I could not find in open sources. It is possible that due to the death of the researcher they were simply lost. There was no one to continue and develop the research, and the Ojos Azules breed itself was on the verge of extinction and has been that way for many years.
The lack of a genetic test and, even worse, little progress in terms of its development, is a common problem for breeders of Altai, Topaz and other cats with DBE. The current situation seems outrageous to me; despite the fact that work with DBE is currently one of the most promising areas in felinology, for some reason, geneticists are still more interested in studying the genomes of lions and tigers or, at worst, dogs for which more than one hundred genetic tests have already been developed and new ones are constantly being developed. For cats, at best, there will be 2-3 dozen such tests, and even these often leave much to be desired in terms of their reliability, and, most surprisingly, their demand among breeders and cat-owners. Recently, I was extremely surprised to find on the website of one of the domestic laboratories a brand new test for a cat disease which the vast majority of breeders have not even heard of, let alone come across it in their work! It turns out that both time and resources were made available to search for the mutation responsible for a little-known disease, but on the other hand, there is neither time nor resources available for research which Russian and foreign breeders have been requesting for three decades, and are ready to provide not only samples for research, but also sponsorship.
The absence of a genetic test makes our work extremely difficult; in particular, it generates a lot of groundless accusations and speculation. Without a gene test, no one can reliably find out whether s/he is working with a long-known gene that someone previously introduced into the breed, or whether he has encountered a so-called de novo mutation, which may not match any of the previously known ones. It is also impossible to know for sure whether the offspring of a cat with DBE has inherited the gene and is a hidden carrier (latent). Therefore we cannot reassure those owners who dread DBE that a particular cat will not suddenly give birth to a blue-eyed or odd-eyed kitten instead of the usual green-eyed chinchillas, they would immediately give it away for free to anyone, just to rid their cattery of such a nightmare.
Considering the circumstances of Nadeya's birth (the father sired about 40 normal kittens, the mother was bred by ourselves), we have quite strong arguments in favour of the de novo mutation. Contrary to popular belief, de novo mutation is by no means a rare phenomenon and has nothing to do with "spontaneous generation" or other near-religious concepts that owners try to use when they have little understanding of the real nature of the matter. In particular, they are often dumbfounded by the excessive - in their opinion - number of DBE cases found independently first in one cattery and then in another even though the proprietors of these catteries categorically deny crossing their cats to any other breeds. As a result, conspiracy theories multiply and there are accusations of "highly secret outcrossing to cats of another breed for the sake of obtaining a commercially popular colour" as mentioned at the beginning of this article.
De novo mutations have long been well known. In human medicine, they are greatly important. There are many traits and features found in human children that are associated precisely with de novo mutations i.e. they are not inherited from the parents. In medical genetics, it has long been recommended that the term "mutation" be replaced by the more neutral term "genetic variant" as this does not carry a negative connotation. This is important and timely, as in reality most genetic variants are not harmful, but are neutral.
So there is nothing improbable in the emergence of a de novo mutation by itself, nor in the fact that de novo mutations with a similar phenotypes can occur almost simultaneously in different places and in individuals that are not closely related to each other. We add to this the fact that until the new trait became popular, the birth of such kittens was not advertised in catteries. No matter how interesting and cute the "non-standard" kitten was, it was quietly taken out of breeding and breeders tried not to spread details of its birth to their associates. However, literally over the past five years, kittens with DBE have become very much in demand. Each such kitten instantly becomes an object of mass interest on the part of breeders and owners; they no longer hide such kittens, but, on the contrary, proudly show them off. They try to find signs of the desired trait even where it does not exist; right up to the last moment they keep peering into the childish blue eyes of small kittens, hoping that this time they will be lucky and those eyes will remain blue.
Of course, we have not lost hope that sooner or later a genetic test for determining DBE will nevertheless be developed and then each of us will be able to find out exactly which mutation we are working with and which one is most convenient. We try, as far as possible, to show initiative, interacting with geneticists, to achieve at least some local research in the direction that interests us. It was in the framework of one such study that an approximately 100-nucleotide deletion was found in the PMEL gene (best known as the Merle gene in dogs) and 3 mutant positions in the PAX3 gene structure, which may be associated with the trait of interest to us. In the near future, it is also planned to analyze the MITF gene on a new batch of samples using a commercial kit for DNA isolation, which allows us to work not only with buccal epithelium, but also with hair. We also continue constantly to search for institutions, laboratories, and specialists who are ready to cooperate in the study of DBE in cats. We collect samples for research, create and maintain an electronic database of pedigrees, and store extensive illustrative material on each cat in our DBE family.
IV. Some information and statistics
To date, we have already clarified many points relating to the features of inheritance and manifestations of our DBE. In many ways, its behaviour is similar to that of other types of DBE, but there may be differences.
1. The phenotype of our DBE appears as blue- or odd-eyes, often with a small amount of white spotting. We can suspect the presence of DBE in a kitten of a non-colourpoint as soon as it opens its eyes: the pupils will glow red, as in colourpoints. Normal green-eyed, genetically black kittens without DBE will initially have no glow, and with age, they will appear green/yellow/blue, but not red. Sometimes the glow of one or both eyes can be faint and hard to spot (when it is possible to see it only in only one out of several dozen photographs); over time, this glow weakens and disappears, and the eyes do not remain blue, but change to their usual colour and acquire the usual yellow-green glow.
2. Our DBE has variable expression. This means that even if a kitten has inherited DBE, it will not necessarily be blue-eyed or odd-eyed. In heterozygotes obtained from mating a DBE cat with a non-DBE cat, we encountered the following variations in the manifestation of DBE (the table also shows the observed percentage of each phenotype to the total number of kittens with DBE). Taking into account that the percentage of DBE cats in a litter when mating a DBE heterozygote to a non-DBE cat is approximately 50%, it is possible to calculate the percentage of kittens of a certain DBE phenotype from all kittens born from such a mating: it will be exactly half the value given in the table.
|Variant of DBE-phenotype||Percent of kittens|
|Blue eyes + little white (fingers on pads, white spot on nase ets.)||33%|
|Blue eyes without visible white||20%|
|Odd eyes without visible white||13%|
|Odd eyes with sectoral staining (part of the eye is blue, part is green) + a small amount of white||7%|
|"Red-eyed" latent (a normal cat without white, whose pupils glowed red in childhood)||7%|
|Green eyes + visible white||7%|
3. In total, we got 53% of kittens with blue eyes, 34% were odd-eyed (of which about a fifth had sectoral heterochromia), and 14% were latents. 60% of kittens had visible white, 40% did not have visible white, and about one fifth of those these 40% are latents, which have neither visible nor blue/odd eyes.
4. Our DBE is inherited dominantly. This means that a DBE cat can be bred to any unrelated non-DBE cat, and will produce about 50% of kittens with DBE and the corresponding phenotypic traits from among the above table. In our case, out of 45 kittens obtained from mating DBE cats with ordinary cats, 21 kittens had certain signs of DBE, and 24 did not (while two of them were colourpoints, which made it impossible to confidently include them in either the DBE or non-DBE group). Even in such a small sample, the ratio is obviously classic Mendelian inheritance: mating a heterozygote for a dominant gene to a cat without the gene gives the heterozygotes and ordinary cats in a 1: 1 ratio.
5. When homozygous, our DBE appears to cause deafness. However, the data is still insufficient, since very few putative homozygotes have been obtained and we cannot 100% confirm that these are homozygotes. Therefore, we would recommend that novice breeders work with our DBE as they do with Fold: only produce heterozygotes and do not take risks until we finally figure out what can be expected here and with what probability.
6. One of the most difficult and controversial issues is the influence of DBE on the cat’s face. Such a feature does exist, however, like all other phenotypic manifestations of DBE, it varies from zero to a certain conditional indicator of "maximum expression". Our DBE in this sense is no exception.
Most often, in cats with DBE, there is a semblance of telecanthus. I will dwell on this in more detail.
Telecanthus in medicine refers to so-called minor developmental anomalies - unusual morphological features that do not have serious medical or cosmetic consequences. Unlike malformations, such anomalies are not accompanied by dysfunctions of an organ or system. Telecanthus is not a "nose bridge problem" as some breeders believe. The nose bridge of cats with telecanthus is not "depressed", nor "flattened" and in general is not deformed in any way. The reason for the development of telecanthus is the stretching and lengthening of the medial ligaments of the eyelids, which leads to a lateral (away from the middle, sideways) displacement of the inner corners of the eyes; while the interpupillary distance remains normal. In humans, this characteristic has a quantitative indicator, an index, which is calculated as the ratio of the intercanthal distance (the distance between the inner corners of the eyes) to the interpupillary. Normally, the intercanthal distance is equal to the distance from the inner to the outer corner of the eye, i.e. the width of the eye. The average interpupillary distance in humans is 60-62 mm, which corresponds to an intercanthal distance of about 30-31 mm, thus the standard value of the index is about 0.5 (considered the norm = 0.6). If the intercanthal distance is significantly greater than the width of the eye, and the index exceeds normal (> 0.6), people can talk about the presence of telecanthus. Calculations for cats have probably not been carried out, but those who wish to do so can make the first measurements and start collecting statistics right now.
In the literature, it is noted that the telecanthus tends to decrease as they grow older. Our experience fully confirms this. Most often, the telecanthus can be seen in DBE kittens at a very young age, that is, before and immediately after they open their eyes; later on, it either completely disappears, or leaves behind signs that are so subtle that they can no longer be so unambiguously associated with DBE: for example, a sloping upper eyelid, which can also periodically occur in cats without DBE.
It is not possible to accurately identify the carrier of DBE only "by the face". We tried, but failed, as it was very desirable for us to identify color points without white as carriers of DBE.
7. The behaviour, and often the very manifestation of any gene in the phenotype, depends on the genetic background i.e. non-allelic genes that interact in different ways with the gene being studied. For example, for mice, whose genetics have been thoroughly studied in great detail, for practically every known gene there is information about its behaviour in various genetic backgrounds. In working with DBE, as well as in working with Fold, any breeder seeks to select such a genetic background that would provide the optimal expression of the colour- or breed-forming gene in the phenotype. For us, this is - apart from getting real British muzzles without pronounced telecanthus, - minimizing the amount of white in the coat, and in future, the stable production of silver and gold chinchillas without white; evidence of white spots will be considered a defect, preference being given to cats without any visible white.
To begin with, the genetic background of British chinchillas is quite favourable in this respect. Chinchillas are a product of long-term selection for the absence of white in the coat (since even a simple medallion is a disqualifying trait in that breed). Even during inbreeding our chinchillas have never been seen to produce individuals with "medallions", determined, presumably, by one of the alleles of the W series (KIT gene) and sometimes found in purebred lines of solid-coloured British cats. However, for prevention, our DBE cats have been tested for all known variants of white spotting (Ws, wg) and we are constantly monitoring this situation. Probably due to a rather favourable genetic background, the amount of white spotting in our DBEs in the vast majority of cases is insignificant and is expressed, as a rule, either in the appearance of 1-4 pink toes on the hind feet, or as a white spot on the muzzle. At the moment we are faced with only one case of kittens born with full white "socks" and never got a kitten with "stockings", while kittens without visible white are born often. They differ from ordinary silver and gold chinchillas only in eye colour.
8. It is our plan to achieve not only the development of a genetic test for DBE, but also achieve the recognition of the new colour of British chinchillas associated with DBE by the leading felinological registries (blue-eyed and odd-eyed chinchillas 61 and 63). To recognize a new colour in an existing breed according to the WCF system, at least 15 individuals of the new colour must be submitted to a special commission. It is unlikely that we will be able to collect so many DBE cats on our own. Therefore, we invite the cooperation of breeders who are seriously interested in the dominant blue-eyes gene and who are prepared for accurate, competent work with it; both with us, on the basis of temporary or permanent co-ownership of cats, and independently of us in their own cattery. Naturally, in either case, we guarantee our collaborators and like-minded people all necessary advice and assistance.