Of all the genetics questions I get asked, no other genetic phenomenon seems to inspire curiosity quite like our cats that have white on them. Some are totally white (epistatic white gene), some have patches of white (the piebald gene), some have cute little white mittens and shoes (gloving),and some have no more than a few little hairs of white on their chests or tummies (lockets). Strangely enough, although all of these cats may be very different genetically, they exhibit the results of the same physiological phenomenon.
To understand this, we have to not only know some basics of genetics, but some concepts of embryology as well. Most of us are familiar with the terms "dominant" and "recessive". The piebald gene, responsible for our bicolors and vans, is dominant. The form of white spotting seen in Birmans in also dominant, but doesn't always show up in every cat that inherits the gene, a characteristic called "incomplete penetrance." If a cat has white spotting, you know that at least one parent had white on him. The same is true of the epistatic white gene. When we see a beautiful shimmering white Persian we know that at least one of his parents was white. The term "epistasis" refers to the fact that the cat's "whiteness" covers up whatever color he may be underneath. For all you know, that white Persian is genetically a shaded tortoiseshell, or a blue lynx point, or a dilute calico.
Two white cats can produce a non-white cat. This is because in order to be white, the cat only needs ONE white gene. All of our genes are inherited in sets of two, one from each parent. Each gene can come in only one or in many "flavors" that are called "alleles". The epistatic white gene has two alleles: W (white) and w (non-white). By convention, recessive alleles are given lower case letters, dominant alleles upper case. If only one parent was white, then it received a white gene (W) from one parent and a "non-white" gene (w) from the other. Since W is dominant over w... the cat is white. Such a cat is called "heterozygous". "Hetero" means "different", and in this case the cat would have the genotype Ww. Either a W or a w would be passed to each of that cat's offspring. If both of the parents were white, the cat COULD (but not always) get a W allele from each parent. This creates what is referred to as a "homozygous" white cat. "Homo" means "same". This cat would have the genotype WW and ALL offspring from this cat would be white, as every kitten would receive a W even if the other parent only gave him a w. When a Ww cat is bred to a Ww cat, the odds are that 25% of the kittens will be WW, 50% will be Ww, and 25% will be ww (non-white). The WW cat will look just like a Ww cat, therefore two cats with different genotypes (genetic make-up) share an identical phenotype (appearance).
This is NOT the case with the piebald white spotting gene, the one that gives the cat fancy its lovely bicolors and tricolors. In this case, the dominant gene is extremely variable in its expression. A cat with only one copy of the dominant allele (genotype Ss) may only have a small amount of white or could have a large amount of white. Cats with two copies of the dominant allele (genotype SS) frequently have very large amounts of white and are referred to as "vans". Carried to the extreme, a homozygous cat may be completely white! This very rarely happens though.
The extent to which the amount of white is expressed can be controlled by selective breeding. By breeding only the cats with a lot of white on them you can decrease your chances of getting cats with very little white. This is because for every "major" gene we know about, there are many little genes ("polygenes") that can influence what our cats look like. For instance, ear size, coat texture, and temperament are not controlled by major genes, but can be influenced by selective breeding and the accumulation of desired polygenes. By the way, older articles on bicolor breeding in Persians warn against getting "too much white". Interesting historical observation, as at the CFA Persian Breed Council Meeting it was agreed by the majority of breeders that you can never have TOO much white!
The opposite direction is taken by Birman breeders. They have selectively bred their cats for many generations to influence the way that the piebald gene is expressed. Through careful selection of breeding stock they can produce cats with white limited only to the four paws with a level of consistency that is quite amazing. The hybridization of Birmans with other breeds (both intentional and accidental) demonstrates that the trait of white spotting is definitely caused by a dominant mutation in this breed. However, this "gloving" mutation, recently identified by researchers as a mutation in the KIT gene, has also been described as a recessive mutation, since the specific pattern of the show quality Birman requires the cats to be homozygous for this allele. The correct pattern can be lost very easily, reintroducing a whole spectrum of distribution of white, once a group of cats becomes heterozygous at this gene. When Birmans were bred to a flame point Himalayan to introduce the red factor into some lines, the gloving was reinstated within a few generations by recreating cats homozygous for the gloving pattern mutation.
Another example in our spectrum of white is the case of "lockets" seen in some cats. These are genetically solid cats with no bicolored or white cats in their background that have a small patch of white hair somewhere on them. This does not appear to be influenced by any major genes, although a predisposition for lockets can "run in lines". Removing cats that throw locketed kittens from breeding programs can lower the incidence. Using bicolored or white cats in a breeding program will NOT cause lockets! However, keep in mind that historically if a cattery began having solid colored cats with lockets, a white or bicolored breeding program would "cover up" the problem.
When these lockets occur, they are almost always seen on the underside of a cat. Spots at the groin or chest are the most common. Now picture a bicolored cat. These cats always have white chests and tummies. You never see a bicolor that is black on the bottom and white on top, do you? White kittens when they are born frequently have a patch of color on them. Where is it?? On the topmost region of the cat: between its ears. Are you getting the picture? White on bottom, color on top.
When an egg is first fertilized, all the cells are the same. This is followed by a process known as differentiation, where individual cells start to take on their unique "duties" in the complete animal. In this process, some cells start to move from the part of the embryo called the neural crest (at the top of what will be the kitten), down over the sides of the embryo towards the bottom. These cells will later further differentiate into cells responsible for many functions, including the melanocyte cells that give color to a cat's skin and fur. If these cells contain the genotype ww and no dominant alleles of the piebald gene they will travel all the way to the bottom of the embryo. If they contain one or two copies of the piebald gene, they can get "lazy" and stop before they complete the journey. If they have one or more copies of the epistatic white gene, this journey will be interfered with as well, stopping very shortly after it starts.
Many factors can influence this "pigment parade". Some bicolors have perfectly symmetrical markings, some have their white "askew". Some have one well defined unfragmented area of white, others have color that seems to scatter across an area. This could be due to polygenes or even possibly what is surrounding the embryo as it develops. In a study of the effects of ultrasound on developing human babies no ill effects were found. However, they did find that more babies were lefthanded if they received ultrasound treatment prior to about 12 weeks. Science has a lot to explore in the field of embryology and environmental affects on developing cells!
Can there be bad effects from stopping the parade of pigment forming cells prematurely? Yes, and this is what leads to deafness in some white cats and (very rarely) in "van" bicolors as well. As I stated earlier, these marching neural crest cells have more than one function to perform. Another type of cell can be formed from these neural crest cells that is essential for sound hearing. That is why deafness is most common in blue eyed white cats and when it occurs in vans, only in those with VERY little color.
The pigment cells are responsible for giving our cats colored eyes as well as colored skin and fur. An eye without these cells is blue. If the pigment cells have marched down from the neural crest far enough to give the eyes color, chances are they have progressed far enough to provide sound hearing as well. But remember how some bicolors are assymetrical? The same thing can happen in whites, causing odd eyed cats or deafness in one ear. I have seen bicolored cats with eyes that are blue on the side towards the middle of the face and copper on the side towards the outside of the face. Now THAT is an odd-eyed cat! As breeding programs consistently produce bicolored and van cats with more and more white on them, blue and odd eyes will increase in frequency in these programs.
Most breeders know that if a white kitten has a "kitten cap" that the kitten is probably sound of hearing. The kitten cap shows us how far the melanoblast cells have traveled. Most of these cats are copper eyed. When I initially started researching this subject, I was under the misunderstanding that all blue-eyed cats were homozygous and copper eyed cats were heterozygous. This is not true. A kitten with only one white parent can still have blue eyes and/or be deaf. The expression of the white gene is highly variable, however, the tendency towards blue eyes (and deafness) can run in lines. The more blue eyed cats in the pedigree, the greater your chances of experiencing blue eyed and/or deaf kittens. Homozygous whites tend to have blue eyes more frequently than heterozygous cats, indicating a possible "additive" effect influencing the migration of pigment cells.
In my opinion, kittens with blue eyes AND sound hearing can not be selectively bred for with any degree of consistency. If you are selectively breeding for the pigment parade to stop as early as possible to get those lovely blue eyes you can not avoid running into deafness at some point. Breeding two "sound hearing" blue eyed cats to one another in an attempt to avoid deafness is a misguided approach, in my opinion. If deafness is to be consistently avoided, copper eyed cats (either white or, preferably, non-white) should be used.
Do not fall into the trap of believing that there is a "deaf gene" or a "blue eyed" gene with a simple mode of inheritance. Unfortunately, this is NOT the case. Remember those Birmans? The pigment parade CAN be controlled, and in these cats to an amazing degree, but it would require many, many generations and very careful control of breeding stock, selecting only for degree of pigment progression. If you want to get an idea of how difficult this will be to do in a white program, visit a bicolor breeder! The fun of working with bicolors is the incredible diversity of expression of this gene. But it also shows how difficult it is for us to "paint" our cats with pigment just where we want it!
Most articles that discuss the breeding of whites also mention Himalayan programs using whites. Some breeders have worked towards sound hearing blue eyed cats by incorporating Himalayans into their white Persian breeding programs. If a physiologically "copper eyed" white kitten is produced, having two copies of the recessive "siamese" gene ( a form of albinism) can magically turn his eyes blue without any regard to how far the pigment cells have journeyed. This basically creates a "white point" Himalayan. However, still remember that the white gene is variable in its expression and a blue eyed deaf kitten can also be produced by the "traditional" effects of the white gene. My suggestion to avoid deafness such a program? Use copper eyed white cats for breeding that had large kitten caps. Some white cats even maintain a "kitten cap" in adulthood. These cats probably have the least likelihood of producing deaf kittens.
For completeness, I would like to say that in some Himalayan and Siamese/Oriental lines there is an albinism gene that has travelled through the generations from their Siamese ancestors. This is the one case where a cat can be white without one white parent. This is a simple recessive gene, recessive to the "normal" siamese allele, which is actually a form of albinism itself. I have only seen one albino Himalayan, with very pale blue eyes. Most of these have vision problems, such as nystagmus (eye tremors). In the case of an albino, the pigment parade is not affected at all -- the pigment cells just don't do their job once they get to their destination!
So now that we know that there is no "blue eyed" gene and no "deafness" gene, what about those annoying lockets? This occurs when the pigment cells get "lazy" and stop just short of the completion of their normal journey. This does not appear to be due to one or two major recessive or dominant genes, but like many other traits a tendency to throw lockets can run in lines. Multiple polygenes are probably involved. Use a cat for breeding that had a "locketted" littermate and you'll probably run into the problem again. Keeping good records of ALL kittens born to a breeding program are valuable aid in controlling the incidence of such undesireable traits as lockets.
I had stated that using bicolors or whites in a breeding program would not cause lockets. I would like to backstep a bit on that position. If a breeding program is selectively breeding for large amounts of white on their bicolors, they may also be selectively breeding for factors that can contribute to the "laziness" of the marching melanoblast cells. This is also true of a program selectively breeding for blue eyed whites. This is purely speculation, and I would be interested in hearing from white breeders producing large percentages of blue eyed white kittens. Have you been getting locketted solids? Especially from lines that throw BEW's bred to non-white cats? This may be the root of the "old wives tale" that using whites will improve the amount of white on a bicolor. Using BEW's to improve the amount of white on a bicolor, however, would be far less effective than using a bicolor cat selectively bred for a lot of white! It is my feeling that any influence that a white cat could have on the markings of a bicolor would be very minor at best.
Taking advantage of the polygenes influencing other programs can also benefit someone wishing to increase the incidence of odd eyed whites in their lines. I have heard a theory proposed that adding bicolors to a program will increase the incidence of odd eyes. I think this is possible... if ASSYMETRICAL bicolors are used. Bicoloreds with perfectly even V blazes may even DECREASE the odd of getting odd eyes, in my opinion, because these cats reinforce the trait of symmetry.
Because the mechanisms of how the white gene and the piebald gene work are so similar, some have proposed that the piebald gene is an allele of the white gene. If this were so, then when a white cat with a van (homozygous bicolor) parent is bred to a solid cat, there would never be any solid kittens produced. The white cat's genotype would be WS, the solid parent would be ww, and all the kittens would therefore be Ww (white) or Sw (bicolor). A ww kitten (non-white, non-bicolor) could not be produced. It also would mean that a white cat could never be masking van, only bicolor. Review of persian pedigrees has revealed that whites masking van HAVE been produced. Litters have also been produced from a white (carrying bicolor) bred to a solid (non-white) cat that contain solids, whites AND bicolors, proving that these genes are not at the same locus
I hope that this gives you some insight as to how our cats get white on them. Amount and placement of white on bicolors can be selectively bred for. Copper vs. blue eyes on a white can also be selectively bred for, although keep in mind that deafness and blue eyes go hand in hand! The absence of lockets can be bred for. I hope that by understanding the mechanism behind why our white and "with white" cats look like they do, you can make better decisions for your own breeding program.
I'd like to thank my usual references, Book of the Cat, Roy Robinson, and Dr. Pedersen's Feline Husbandry, as well as those embryology texts in college, long since stacked away somewhere! I'd especially like to thank Laura Lewellen for answering my initial questions on this phenomenon.
Come join an exciting group of cat fanciers interested in feline genetics issues by subscribing to our list using the button below.
If you buy my feline genetics book, Robinson's Genetics For Cat Breeders and Veterinarians, by clicking through the link below, a portion of the sales price goes to support cat health research.