The expression of the colour white in the cat, or, should one say the absence of colour, overrides the genetics of other colours or patterns in the cat. In other words the visual result will be the same whether the individual cat is black, blue, brown, lavender, orange, whether it is tabby, pointed, or smoke and this applies to all breeds. Thus, it is fascinating to compare the occurrence of white in different individuals and to note the range in the quality, placement, effects upon eye colour and more worrisome, on the occurrence of deafness in these white cats. To understand white a little better, here is some simplified information on the origin of white, its mode of transmission and expression as well as its different names.
White
White... a little, a lot, like crazy!
Origin
It is the endogenous virus FERV1 which is responsible for the genetic mutation resulting in the inhibition of colour, perceived as the colour white in the cat, dog, mouse, horse, and even in humans!
A virus? How is that possible?
Viruses, as undesirable as they might be, play a large role in natural evolutionary phenomena and the changes they bring may or may not be beneficial. More specifically, retroviruses have the ability to collect all the elements that they need to replicate from the nuclei of cells they have infected. Thus, to replicate themselves, they take bits of DNA needed from the host and leave behind mutated remnants in which bits of viral DNA have been substituted for bits of regular DNA. These cells, not destroyed by this mutation, in turn reproduce mutated cells.
It is in this way that the retrovirus FERV1 was integrated into the genome of several species. In the cat, it is the origin of the two varieties of white identified on the locus W, one being dominant white and the other, spotted white. The insertion of the complete FERV1 viral genome is associated with spotted white, while the partial insertion of its repetitive terminal line induces dominant white. The absence of pigmentation in the coat and skin is caused by complete penetrance and incomplete penetrance induces the absence of pigmentation in the eyes and alters the sense of hearing.
Characteristics
The coat pattern called “dominant white” is characterized by an absence of coat pigmentation in adult cats and normal to altered eye pigmentation. Certain specimens will retain their yellow eye colour while others will have blue eyes. This phenomenon interests breeders who sometimes obtain white or partially white cats having one yellow eye and one blue eye, a phenomenon commonly known as “odd eyed”.
In certain cats, dominant white has long known to be associated with incurable congenital deafness. A German study showed that in a sample of 84 cats with white coats, 20.2% were deaf. Moreover, cats having blue eyes are more frequently deaf than cats having gold or green eyes. (2)
Hereditary Transmission
The amount of white spotting depends on the genetic baggage of the parents. If neither parent has any traces of white, they can only produce kittens without any traces of white (look out for those which hide traces of white, sometimes at the base of the throat and neck). The quantity of white expressed seems proportionate to the sum of white present on each of the two alleles.
As far as the dominant white is concerned, transmission is dominant, meaning that only a single copy of the gene is necessary to obtain white cats. Certain cat breed clubs frown upon breeding of two white cats, to avoid getting double copies of the W gene since the probability of deafness is multiplied.
Here are different coat patterns:
Figure 1:
Source: Endogenous Retrovirus Insertion in the KIT Oncogene Determines White and White spotting in Domestic Cats, G3 (Bethesda). 2014 Oct; 4(10):1881-1891.
To find out more, consult the following document:
http://documents,irevues.inist.fr/bitstream/handle/2042/48209/AVF_2012_3_205.pdf?sequence=1
References:
[1] DAVID Victor A. et al, ENDOGENUS RETROVIRUS INSERTION IN THE KIT ONCOGENE DETERMINES WHITE AND WHITE SPOTTING IN DOMESTIC CATS, Octobre 2014, dans Genes/Genomes/Genetics, volume 4,
Endogenous Retrovirus Insertion in the KIT Oncogene Determines White and White spotting in Domestic Cats, (page consultée le 5 février 2019)
[2] ABITOL Marie, Bull. Acad. Vét. France-2012-tome 165-No3:
ABITOL Marie, GÉNÉTIQUE DE LA COULEUR ET DE LA TEXTURE DUE PELAGE CHEZ LE CHAT DOMESTIQUE, 12 avril 2012, dans academie veterinairedefrance,
http://documents.irevues.inist.fr/bitstream/handle/2042/48209/AVF_2012_3_205.pdf?sequence=1 (page consultée le 1 er mars 2019)
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