White Boxers and Deafness
by Bruce Cattanach
Having just written a long review article on white color and deafness in
Dalmatians for the British dog press I should make a comment on this situation
The terms whites and checks appear to be being used interchangeably in recent correspondence. They are different. The white Boxer may have pigmented patches around the eyes and ears and other limited points on the body, but the check, as shown in old time photos has much more pigment and could be called piebald (50:50).
The white Boxer carries two doses of the extreme white spotting gene, s-w (s-w/s-w) and is produced by the so-called flashy animals, which, in the UK Boxer, carry one dose of the gene.
So, here, all our show Boxers are carriers. Crossing these together gives 25% whites, 50% flashy and 25% solid colored.
In the classic work on coat colour in dogs by mouse geneticist CC Little, some flashy US Boxers were thought to carry a different form of the white spotting gene, s-I (Irish spotting). They would have two doses of the gene, like Basenjis or Boston's. I have not found any evidence of this form in UK Boxers.
If you find real checks appearing in American Boxers, then you may have this s-i form of the gene still present. These would be compounds of the s-w and s-i, as demonstrated recently in my cross of a s-i/s-i Corgi with a white s-w/s-w Boxer.
I might add here that there is some movement at the UK Breed Council level to recognize that the flashy show Boxers all carry the gene for white and accept that breeding them together to produce whites will soon be considered unethical. Solid Boxers may be promoted both for showing and breeding.
Whites are commonly put down here too, not just because of the risk of deafness but because they are so difficult to home happily. They are bought cheap and regrettably are liable to be treated cheap, although many do find ideal homes.
As to deafness, the genetic basis of white in Boxers is the same as in Dalmatians, albeit without the ticking factor to give the spots. In the UK the incidence of deafness in Dalmatians is about 5% bilaterally deaf and 13% unilaterally deaf, total affected 18%. In the States according to Strain the figures are somewhat higher, 8% bilateral and 22% unilateral, total affected 30%. I do not know of any good figures for Boxers but it would be reasonable to believe that the incidence is similar. Only the bilaterally deaf Boxers would be recognized of course; under 10%.
The cause of the deafness associated with the white colour is the absence of pigment cells in the inner ear resulting in a loss of sensory hair cells at about 6 - 8 weeks of age. The shortage/absence of pigment cells is also the cause of the white coat and unpigmented third eyelids (haw). Generally speaking, the more pigment in the coat the lower will be the risk of deafness, but all predominantly white dogs are at risk of being deaf.
Please note however that not all white dogs are white because of a lack of pigment cells. Some like West Highlands and Poodles just have extremely diluted pigmentation; they have a full complement of pigment cells, so are not at risk of being deaf. Biscuit shading, commonly around the ears and along the back can distinguish this form of white coat.
There are of course many other causes of deafness in dogs, people, mice etc. Very many deafness genes are known in mice. Not all are attributable to the absence of pigment cells. And deafness can be caused by external factors too.
Fawn - All Boxers have a fawn base coat. The brindling pattern, and white color markings are modifiers of this base coat. Dogs appearing fawn do not have the genes for the brindle modifier. Two fawns bred together will always produce 100% fawn puppies (leaving the white markings out of the equation for now).
Brindle - Brindles have a fawn base coat, but also a modifying gene that produces brindle striping. The stripes may be sparse, and far apart, or so large and numerous that the dog appears black, especially when very young, or from a distance. The dog may have one or two genes for the brindle modifier.
If the dog has two genes for brindling (the dog is said to be "homozygous"), then they will always produce only brindle puppies, even if bred to a fawn (again leaving the white markings out of the equation). A dog with one brindling gene, and one non-brindling gene ("heterozygous"), that is bred to a fawn, on average, will produce 50% fawn and 50% brindle. Two brindles that are heterozygous for the brindling gene are bred together, then on average they will produce 25% fawn, and 75% brindle. Furthermore, 1/3 of the brindles (25% of all puppies) will be homozygous brindle, capable of producing only brindle puppies. A heterozygous brindle bred to a homozygous brindle will produce all brindle puppies; half will be homozygous, half-heterozygous. And of course, the fawn-homozygous brindle mating would produce 100% brindle heterozygous puppies - brindles capable of producing fawns.
White marking - White markings are controlled by a gene that is totally separate from the base color. White Boxers often have fawn or brindle spots, indicating whether they would have had a fawn or brindle base color, had not their white markings been so extreme.
There are some disagreements about the actual genes involved in white markings in Boxers, but basically, "plain" or almost solid color Boxers, are believed to have either no genes for white markings, or as having genes for the "low end" of the white marking scale. White Boxers are seen as having two genes for the extreme of the usual white marking pattern. Flashy Boxers are seen as having one gene for no white markings (or low-end markings), and one gene for extreme white markings.
Two white Boxers will always produce 100% white puppies. Exacted percentages when two flashy Boxers are bred together are 25% plain, 50% flashy, and 25% white. When two plain Boxers from show lines are bred together, they are expected to produce 100% colored (non-white) puppies, though markings may vary.