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.
References
Anderson, H
et al, Acta oto-laryng. Suppl. 232, (1968).
Cattanach, BM
Genet. Res., Camb. 23:291-306 (1974).
Famula, TR et al,
Mammalian Genome 7:650-653 (1996).
Greibrokk, T J. Amer Anim.
Hosp. Assoc. 30:170-176 (1994).
Grüneberg, H In: The
Genetics of the Mouse, The Hague, Martinus, Nijhoff (1952).
Hayes, HM
et al J. Amer. Hosp. Assoc. 17: 473-476 (1981).
Holliday, TA
et al, J. Vet. Intern. Med. 6:166-174 (1992).
Lyon, MF Phil. Trans. Roy. Soc. 259: 141-152 (1970).
Mintz, B and Russell, ES
J. Exp. Zool. 134:207-237 (1957).
Robinson, R In:
Genetics for Dog Breeders, Pergammon, Oxford (1982).
Schaible, RH J.
Exp. Zool. 172:181-200 (1969).
Steel KP Annu. Rev. Genetics
29:675-701 (1995).
Strain, GM Br. Vet. J. 152:17-36 (1996).
Strain, GM et al J. Vet. Intern. Med. 6:175-182 (1992).
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Strain, GM and Tedford, BL J. Vet. Intern. Med. 10: 190
(1996).
Wood JLN et al, BSAVA News pp 559-561 (1996).
Wood, JLN and Lakhani, KH Vet. Jour. 153:000-000 (1997).
Yuzbasiyan-Gurkan, V. American Kennel Club, Molecular Genetics Canine
Genetic Health Conference, October 1994.
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