by Isabelle Bellavance
The color white, or, to be technical, the lack of color in certain areas, seems to have a number of genetic explanations. Some are dominant and others are recessive. In actual fact, white is not a color because it is caused by color or ‘pigment cells’ that where insufficient in their
numbers to make it all over the cat’s body. Let me explain…
During the development of the embryo there is initially only one cell. This cell contains all the genetic material originating from each parent that
will dictate the future appearance and metabolic functions of the new organism. This cell then goes on to divide repeatedly into more identical cells. As the number of cells increase, they also become specialized in their functions, activating certain parts of their genetic code while other cells that will have other specializations activate the rest. The cells that eventually divide and become the pigment cells start at the top of the embryo, which is called the neural crest. As their numbers increase, they ‘march’ down so to speak, almost like hot chocolate fudge being poured on a scoop of ice
cream. The genes, which actually produce the white, code to alter or deactivate some of the pigment cells. Depending on how strong the white gene is, it can have a greater or lesser effect in how many pigment cells are left to travel from the top of the embryo towards the bottom.
This is why, it is more common to see white at the extremities of an animal, almost like there was not enough fudge to finish the job, or the ice cream was too cold and the hot fudge froze too quickly. When comparing animals with varying amounts of white, it seems to be seen higher and higher on the body – i.e. feet, before legs, tummy, face, back, and lastly top of head.
There are many genes that could produce the white. In the Ragdoll, we are working with the White Spotting Factor. We will see that it varies enormously in its appearance. The white spotting factor (WSF) has always been assumed to have a mind of it’s own and be uncontrollable. Yet there is only one gene at play here, and we know it’s dominant in it’s expression, but there are such extremes in what it can do… This apparent erratically is probably why most breeds have elected to label it « and white ». However, breeds like the Birman and the Ragdoll have been able to control it to some extent. The fact that our Ragdolls white does not breed « true », as we can have all three patterns (Colorpoints, mitted and bicolor) in the same litter, seems rather disconcerting, though, to those outside the breed.
The Birman and their white gloves were long thought to be a separate and recessive ‘gloving’ gene. Then it was recognized that it was
indeed part of the WSF, but that generations of careful selection had arrived at a tight control of it’s expression, which is why the mitts are fairly
Roy Robinson, in his book of feline genetics, came up with the theory that the white spotting factor had many ‘grades ’ of expression.
He categorized them roughly in 10 levels that increase in the expression of white – going from just a few hairs to an all white cat, commonly called
the ‘Dominant white’.
For the sake of comprehension, we will identify the grades that apply to the patterns we see in the Ragdoll.
Grade 2 – mitted
Grade 4 – bicolor (we have two kinds – high mitted and true bicolor)
Grade 6 – mid-high white
Grade 8 – high white/van
Of course, in the past, the grades 4, 6 and 8 have all been registered as bicolor. Hopefully, after reading this article, breeders will start taking
notes and be able to recognize the genetic grade of their cats so as to then be able to better predict the levels of white in the offspring. This does
not mean that we will start registering our cats in any other than the three current patterns, but simply taking more attention to these details and understanding their implications in the offspring.
The Birman expression of white will be discussed here, simply as a means of comparison. There is a very low level of white – Grade 1 that does not
appear to express itself. This might explain the recessive assumption for the Birman mitts. You may be able to find a few white hairs – in between
toes is a common place to look for them, but when a cat has two copies of this low grade, they add up to a Grade 2, which resembles our mitteds in terms of amount of white. As the WSF series are all dominant, perhaps, if there is a very small amount of white in the heterozygous grade 1, it could go unnoticed or may be considered to have incomplete penetrance. You are not likely to see the expression, unless there are two copies, and then you will see the mitts. If all Birmans have two copies of the Grade 1, as each parent MUST give one copy to their offspring, all offspring end up with two copies of the Grade 1 also. Since 1 + 1 = 2, then all the babies are mitted…
This will hopefully also convince the skeptics, why it is so firmly believed that the « Birman type » that was the foundation of our breed was NOT a Birman as the Ragdoll genetic is so different…
The following is a punnent square that shows the breeding of two Birmans and how such a conclusion of the behavior of their WSF was derived:
Father (S1 S1) S1 S1 Mother S1 S1 S1 S1 S1 (S1 S1) S1 S1 S1 S1 S1
As you can see above, four out of four possible offspring would be S1 S1, thus these two add up to a Grade 2 and produce a cat that has white mitts. The reason I say that the two grades add up is that although this gene is dominant, it seems to have an additive effect to produce the observed level of white. The various levels of white are static, like different versions of the same gene, and a cat that has two Grade 1s, cannot suddenly pass on a Grade 4 to its offspring.
The Ragdoll breed is particularly interesting to work with to understand the functioning of the WSF, as our cats are more or less categorized according to the amount of white they have. The colorpoints don’t have any white at all, the mitteds have it limited to the feet, and the bicolors have a larger portion of the underbody and legs that are white. As the first geneticists to look into the heredity of the white in our breed were in the USA, it was believed that we only had three patterns and that our mitted gene worked as follows:
Father mitted) (S2 s) S2 s Mother mitted) S2 S2 S2 S2 s (S2 s) s S2 s s s
The above punnent square describes what we see in breeding between two mitted cats. Large ‘S’ was used to illustrate that the White Spotting
Factor is dominant – so the little ‘s’, indicates the recessive form – thus NO white. The results above show that out of 4 kittens, 1 (25%)
would be a bicolor, which we will call a high mitted – it has two mitted genes that add up… 2+2=4, two (50%) would be mitted like their parents,
and one (25%) would be a colorpoint.
This also implied that a bicolor (of the high-mitted variety), bred to a colorpoint produced a whole litter of mitteds:
Father (high mitted) (S2 S2) S2 S2 Mother (colorpoint) s S2 s S2 s (s s) s S2 s S2 s
There have always been a few rare cats that did not completely follow this and they were always sort of disregarded, as their inheritance did not follow ‘the rules’. By some fluke, such a cat was part of the handful that were sold by the Dayton’s to Europe for them to start up their breeding
programs. Because the genepool was so small, and this cat was so great, the genetics were somewhat biased towards a different kind of genetic bicolor. This was termed the « true bicolor ». As it was first described in the first book written on Ragdolls, which was, incidentally, written in England… The terminology for this pattern was retained but it is not truer than any of the other variations, just works differently. This pattern produced the best kittens when bred to a colorpoint:
Father (colorpoint) (s s) s s Mother (true bicolor) S4 S4 s S4 s (S4 s) s s s s s
As you can see, 50% of the kittens would be ‘true bicolor’ and the other half would be colorpoints.
The following is a chart of the various patterns we have in the Ragdolls, and hopefully a means for breeders to understand the heredity and potential outcome of breedings. The punnent squares for each of the breedings have been worked out to provide you with a quick and easy reference to the potential for different patterns in your litters. You will notice that some combinations can produce different ‘genetic’ bicolor within the same litter. Because the phenotype or outside appearance of the various bicolor patterns cannot easily be distinguished due to much overlap between their phenotype, it is recommended to keep track of the genetic pattern of our breeders. Ideally it would be to keep them separate, and work on controlling the exact distribution of the white by segregating generation after generation for polygenes that regulate the precise distribution and symmetry of the markings. However, because one cannot afford to disregard genetic diversity for the sake of pattern, it may mean that such a breeding is the best thing to do. Ideally, the insuing offspring should then be bred to a colorpoint so as to « split » the S4 and S2 genes, or you may have to use a colorpoint to « discover » what kind of genetic bicolor you have and work from there.
At this time, it is unknown where the blaze of our mitteds and the occasional white tail tips such as that present in Daddy Warbucks, originate. It
was thought that it may be the expression of a locket, but this would imply that we could occasionally see it in colorpoints too. There has
not been any evidence of this. However, it often seems to run in lines. Another assumption is that it is indeed a recessive expression of a locket,
and these genes would be associated with the S2 gene. By associated, I mean that it is located near the WSF gene, on the same chromosome so that they would always be inherited together. The next assumption is that its expression is controlled by polygenes that alter a little the expression
of the white in the mitteds. Lastly, it could be the expression of yet a different level of white…. but that will be for another story… 🙂