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White Coat Colour and Deafness.

Test for other factors

Strain (1992) has screened for other factors which might influence the incidence of deafness.  These included sex, colour (black, liver, lemon, tricolour),retinal pigmentation, eye rim and nose pigmentation, spot size and level of marking.

Inconsistent results were obtained for several of these characters at the three American test sites tested (Table 2), but only retinal pigmentation (in addition iris pigmentation and presence of patches) showed an association with deafness.

Sex differences have been suggested (Holliday 1992; Wood & Lakhani 1997) but have not been observed in the other studies cited.

Breeding data and deafness

Many investigators have noted that Dalmatians with normal hearing in both ears, as shown by BAER testing, produce fewer affected puppies than those showing evidence of deafness (Strain 1992, 1996; Yuzbasijan-Gurkan 1994, Tedford 1996; Wood and Lakhani 1997).  Some of the key data are presented in Table 5.

In Yuzbasiyan-Gurkan's (1994) study upon American dogs the incidence of bilaterally deaf dogs was three times higher among the progeny from unilaterally affected x normal crosses than from normal x normal crosses.

More detail is provided by Strain and Tedford's (1996) study which shows that both bilaterally and unilaterally deaf puppies to be much more common from affected x normal matings than from normal x normal matings.

A similar result is suggested in Wood and Lakhani's (1997) smaller study upon British Dalmatians.

Such findings provide the evidence that deafness of white dogs, and Dalmatians in particular, has a genetic component.

As has already been mentioned, however, it is well-established that the amount of white coat associated with the s gene in both mice (Schaible 1969; Gruneberg 1952) and dogs (Robinson 1982) readily responds to selection.

Since this change in the coat results from a change in the numbers or migration of pigment cells, it would be surprising if this effect did not extend to the inner ear to give a correlated increase or decrease in the incidence of deafness.  Such changes in effect simply reflect shifts in expression of the s gene brought about by modification of the "genetic background".

There is no need to hypothesise separate genes for white coat colour, blue eyes, and deafness in white dogs.  The s gene (or M gene in other breeds) is the common genetic causal factor.

Implications for the control of deafness

It should be clear from the above that white coat, especially in the absence of pigmented patches, blue eyes and deafness are intrinsically linked.  All have the common basis of absence of pigment cells.

It should also be clear from the findings described that each type of effect can be modified by selective breeding.

Both the American and UK Dalmatian data show that deafness can be reduced by selection; it has been shown in Norwegian Dalmatians that blue eyes can similarly be reduced; and it has long been recognised both in laboratory mice and dogs that selection can modify the extent of pigmented areas in the coat which in minor degree are seen as patches.

The response to selection for any one of these characters implies that pigment cell numbers and/or migration is being modified.  It follows that selection for any one character will modify the others and this points to the Dalmatian dilemma.

Selection for hearing (whether by BAER testing or DNA approaches), or against blue eyes (Greibrokk 1994), may be expected to increase the incidence of dogs with the pigmented patches.  But, the presence of patches does not accord with the breed Standard.  There is therefore breeder selection against patches, and this means unwitting reverse selection for deafness.

To expect that selection against deafness will lead to the production of hearing dogs without patches is asking a lot.  It means that in some way it is possible to increase the numbers and/or migration of the pigment cells such that there is an increased chance of them specifically reaching the stria of the inner ear but not regions of the skin and coat.

Amazing things have been achieved in dogs by selective breeding but this would represent the hardest of all.  It is rather like expecting to be able to breed si or sp dogs with long white socks on one their forelegs but full pigmenation of the other.  Variations between extent of white on the legs does occur but generally the amounts will tend to be similar.  To change this by selection must be virtually impossible.

A way forward

I understand that Dalmatian breeders are generally giving a significant support to BAER testing.  This, however, records only the one character, hearing.  As  far as I am aware, no note is made of eye colour or patches.

In view of the association between the three characters it would seem wise to record all the data at the same time and consolidate them in a way that breeders can see for themselves the association.   It would not be difficult to produce diagrams that could allow left/right eye colour, and patch size and location to be recorded as well as BAER results on each ear.

The critical question is how to utilise the results to reduce the incidence of deafness in the breed.

Here I would suggest that compromise should be the byword.

There would seem to be no justification at all for breeding from bilaterally deaf dogs and I would imagine that few breeders would disagree with this.  But, what about the unilaterally affected dogs?  They are outwardly perfectly normal, yet the breeding results show that such partially affected animals are likely have more affected offspring than those with normal hearing (Table 5).

It would therefore be most effective to take all of these dogs out of the breeding population.  However, they are too numerous, making up perhaps 20% of the Dalmatian breed.  Such stringent selection is too severe to be tolerated.

A compromise solution might be to give some leeway to bitches according to individual breeder needs but treat stud dogs more rigorously.   However, anything that would favour the truly normal dogs for breeding could be considered.

The data accumulated further indicate that risks of deafness can be reduced by discarding blue eyed dogs from the breeding population, and I suspect few UK breeders would find any difficulty with this.   However, the more controversial issue is the patching.  

Selection against patched animals must already be a burden on the breed, as well as enhancing the risks of deafness.   Surely there must be scope for compromise. 

Were limited patching around the ear or eye made acceptable within the Standard, the incidence of deafness might drop as low as that found in white Bull Terriers (Table 1) and there might be scope for further improvement by selection.

This would virtually eliminate totally deaf animals from the breed, which is the most that Dalmatian breeders can realistically hope for and all that is essentially needed.


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