The white spotting gene is a highly variable, difficult to control gene, which produces white spotting randomly, with any colour and is therefore, a separate entity as such.
Major genes are those which affect dominant and recessive traits, such as colour, pattern and coat type. Polygenes, are the many minor genes, which are responsible for the degree of white spotting, amongst other things. These cannot be manipulated individually, however by repeatedly selecting for desirable characteristics, they can be controlled to a degree.
Breeding cats with a low degree of white decreases the chances of getting cats with a high degree of white. By careful selection of breeding stock, Birmans selectively bred for many generations for a low degree of white can remarkably reproduce that degree with some consistency. The hybridisation of other cats with the Birman to produce the Ragdoll, has indicated that gloving can be lost very easily, with a whole new spectrum of the distribution of white being introduced.
To understand this process we need to look at how the white spotting gene works. When an egg is first fertilised, all cells are the same. This is followed by a process know as differentiation, where individual cells start to take on their unique functions in the complete animal. In this process some cells flow from the top of the embryo called the neural crest (spinal area), down over the sides of the embryo toward the feet. These cells will further differentiate into cells responsible for many functions, including the melanocyte cells that give colour to the skin and coat. If these cells contain the genotype for the absence of white or white spotting, they will travel all the way to the feet of the embryo (full colour). If they contain one or two copies of the spotting gene, they can get lazy and stop just before they reach their final destination, the feet and under carriage (low grade white spotting). If they have one or more copies of the epistatic (masking) white gene, the journey will be stopped shortly after it starts (completely white).
While there has been much speculation regarding the gloving gene, with some thought that it is separate to that of the dominant white spotting gene (responsible for Bi-colour and Van patterns), there has been no conclusive evidence to support this theory.
The knowledge gained by experimental programmes involving crossing out to other breeds, to introduce new colours, has led to the belief that low grade white spotting is definitely carried as a recessive. First generations may lose the white spotting, when outcrossing but it is rapidly recovered in successive matings.
The white spotting in the Birman is not in fact confined to the feet as most people believe. As with all low grade white spotted cats, white is typically confined in varying degrees to the feet, abdomen, and chest, the Birman is no exception. Genetically, the Birman is a Pointed, low grade Bi- colour as is the Mitted Ragdoll. In this Semi-albino series, white chest and abdomen spots can be difficult to identify, due to the fact that the immature coat of the pointed cat is pale and provides no contrast. On maturity as body toning increases these spots or patches become evident, varying in size to some degree with each individual.
The progression of white spotting results in the reduction of coloured areas and an increase in the white areas. Typically increasing up the legs (note: it is never the other way round, i.e. a white body with coloured legs and abdomen), abdomen, chest, and chin. A Van pattern or completely white cat, which is the full expression of the white spotting gene (masking its original colour and pattern) is the result.
The typical nose and chin involvement of white spotting, as seen in the Domestic and Ragdoll population with regularity, has virtually been eliminated, by rigid selection within the Birman breed.