The Dalmatian, despite the
presence of its pigmented spots, is basically a white dog and, as a breed, is
widely reported to have a notably high risk of deafness (Table 1).
association of deafness with white coat colour has long been recognised,
veterinary researchers have been perplexed by the inheritance of the deafness
component. Thus, it has evident to all that hearing dogs when mated together
can produce deaf pups, but it has also been found that deaf dogs mated together
can produce hearing pups.
Variable sex differences in incidence of deafness
have also been reported. As a consequence recessive, dominant, multi-factorial
and even sex-linked inheritances have been postulated (see Greibrokk 1994;
Anderson et al 1968).
Despite this confusion it has been shown in studies
conducted both in the UK and America that selective breeding for hearing dogs
can reduce the incidence of deafness.
This has been greatly facilitated by
"BAER testing" (brain stem auditory-evoked response) which allows unilaterally
affected dogs (deaf in only one ear) to be distinguished from totally normal
There is therefore the potential for breeders to reduce the incidence
of deafness in their stocks.
Molecular genetic approaches to identify genes for
deafness have also been contemplated in the hope that this might further assist
breeders in selecting dogs that are free of deafness genes.
This report is written on
behalf of the KC/BSAVA sub-committee to present a less complex genetic basis for
the deafness in white dogs; it represents a geneticist's interpretation of the
published findings made both in dogs and laboratory mice.
the basis of the deafness is not recognised in terms of deafness genes, but
rather upon the recognised mechanisms by which pigmented and unpigmented (white)
areas are produced throughout the body, be they in the coat or elsewhere.
Unfortunately the implications create something of a dilemma for Dalmatian
breeders. As already implied by Famula et al (1996) and Wood et al
(1996), they suggest that positive selection for hearing dogs will be offset by
negative selection to meet the demands of the Breed Standard.
The basis of white coat colour
The white coat colour of
dogs can be brought about by one or other of two ways. It may result from an
extreme dilution of the pigment produced by pigment cells such that some degree
of off-white shading may be evident. A breed showing this form of white coat is
the West Highland White Terrier which may often show a "shading" on the ears or
along the back.
The second form of white coat colour results from the actual
absence of pigment cells. This form of white coat is found in breeds in which
distinct patches of fully pigmented coat are occasionally or commonly found.
These commonly occur around the eyes and ears but such distinct patches may also
be found on the body. The white areas or markings on dogs which are
predominantly pigmented can also be attributed to this cause.
At least two genes are
known to cause white areas by the absence or diminution in numbers of pigment
One is the dominant merle (M) gene, commonly carried in the
heterozygous (single dose) form in breeds of Collies, as well as Cardigan Corgis
and Harlequin Great Danes. It is the homozygote with two doses of the gene that
is principally affected, typically being totally white. Because the homozygote
is also liable to be blind and deaf as well as being otherwise severely
impaired, matings that would produce such dogs are usually avoided.
A second gene that reduces the numbers of pigment cells to cause a white coat colour is
the recessive (s) gene, the extreme allele or form of which gives the
near all white coat typically seen in Dalmatians, English Setters, white Bull
Terriers, white Boxers, etc.
There are several
different alleles of the s gene, these bringing about characteristically
different levels and distributions of white coat.
The top dominant allele, S
or +, gives the essentially solid coloured coat seen in many breeds, but minor
levels of white may be found on the toes, chest and belly.
The next allele is that for the so-called Irish spotting (si). Here, the white
markings are principally located on the foreface, around the neck, on the lower
limbs, chest and belly. Breeds carrying this allele are the Boston Terrier and
Then there is the piebald spotting allele (sp) which
has a wider distribution of white as illustrated in some Cocker Spaniels and
And, finally, there is the extreme white spotting allele (sw)
found in Dalmatians and other breeds previously mentioned (Table 1).
In these dogs, almost the entire coat is white (ignoring the pigmented spots for the
moment) but pigmented patches, as previously described, can sometimes also be
found. Elucidation of these pigmented/non-pigmented patterns provides an
important clue for the relationship between white coat colour and deafness.
Studies in laboratory mice
have shown that the pigment cells derive from the neural crest of the foetus.
Prior to birth, they migrate from this tissue and colonize pairs of specific
sites on each side of the head and the backline of the body. Three pairs of
sites exist on the head. One site lies close to the eye, another lies close to
the ear, and a third lies at the occiput, the latter no doubt being the basis of
the Blenheim spot of Cavaliers King Charles Spaniels.
Various estimates suggest
that there are about six sites along each side of the body, with a possible
larger number along the tail (Schaible 1969; Mintz and Russell 1967; Cattanach
1974). At each site one or a very few pigment cells (maybe up to three, Lyon
1970) proliferate to give clones of cells which migrate outwards so that they
join up, but they also spread down each side of the head and body until they
meet up on the underside, and further spread down the legs towards the toes.
The most remote regions (under the chin, chest, belly and lower limbs) are the
most at risk of remaining uncolonised by the pigment cells and therefore white.
This is the most common basis of the white markings seen in many species, dogs,
cats, mice, horses, cattle etc.
The various s
alleles both reduce the number of pigment cells and impair their migration to
different degrees. With the normal S allele, full pigmentation typically
occurs, but the most remote areas (notably chest and toes) are most at risk of
being uncolonised. Hence the occasional white markings on the chest, belly and
toes of solid breeds such as the Irish Setter.
With the si
allele, the initiating sites on the neck may never gain their single pigment
cells and elsewhere pigment cell migration is generally impaired. Hence the
white collar and extensive white markings in such animals.
The same mechanism
may apply more widely with sp to give the piebald pattern, and
with the extreme sw, most of the coat is white with only the
occasional pigmented patch seen in regions close to the original sites, notably
those around the eyes and ears. In all cases the boundaries represent the "tide
marks" of the pigment cell migration. In dogs, this spread clearly continues
after birth, patch size increasing and white areas decreasing over the first few
days or weeks.
While the spread or
migration of the pigment cells produces the characteristic patterns that are so
familiar in dogs, this is not the whole story. This is best illustrated by the
regular occurrence of pigmented spots on the skin of white regions, with the
hair in these spots occasionally also being affected. It is likely that the
ticking (T) factor of Dalmatians only exaggerates this normal phenomenon
to give the spotted pattern that characterises the breed.
There has been much
scientific controversy over the mechanisms responsible but it seems likely that
pigment cell migration initially covers the whole of the body. There is then a
period (before birth) in which most pigment cells in the potentially white areas
fail to survive. Then proliferation and migration restarts (after birth) as can
readily be observed in dogs to give the final "tide mark" patterns seen in si
and sp animals and the appearance in Dalmatians of spots
within the white areas where single pigment cells have survived. However, while
such theory may be important for the genetics of spotting/ticking it probably
has little relevance for the issue of deafness.
It should be noted that
there is a considerable variation in the amount of white coat shown by dogs
possessing the various s alleles, and in both dogs (Robinson 1982) and
laboratory mice (Schaible 1969; Gruneberg 1952) it has been found that the
levels can be readily modified by selective breeding. Thus, starting from an
intermediate level of white, selection upwards can generate near-all white
animals and selection downwards can produce near-solid ones.
There can also be
considerable right-left asymmetry. For example, there may be a pigmented patch
around an eye or ear on one side of the head but not on the other. Beyond this,
with greater amounts of white, one or both eyes may be completely or partially
blue, this resulting from the absence or near-absence of pigmentation within the
Overall, therefore, there is a significant chance element to the pigment
cell distribution. This is important as pigment cells also colonize the inner
ear and play an as yet undefined but essential role in maintaining its function.
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