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This is a short article, kindly written by Professor Strain to clarify one or two of the misconceptions that occur, concerning the research into a link beweeen coat colour and deafness in the Dalmatian and other dogs. A more complete picture of the research and results are on Prof. Strain's web site.
Monday 7th June 1999 Written by George Strain.James Wood's response was correct - deafness is not linked to the dominant ticking gene, which determines the size and extent of spots on the Dal. A study I published in 1992 examining over 1000 Dals found no association between deafness and spot size or heaviness of markings (Strain, G.M. et al. 1992. Brainstem auditory evoked potential assessment of congenital deafness in Dalmatians: associations with phenotypic markers. J. Vet. Internal Med. 6: 175-182.) Instead, it is the extreme piebald gene that is associated with the deafness. This gene is recessive, so all Dals are homozygous, or they would not have white skin and hair. Piebald covers up the underlying black (dominant) or liver (recessive) color, and ticking "punches" holes through the white to show the spot color. The same study found a positive association between (the presence of blue irises and missing pigment behind the retina) and deafness (i.e., blue eyes == more likely to be deaf). It also found a negative association between patch and deafness (i.e., patch == less likely to be deaf.) Although deafness IS associated with the extreme piebald gene, this is not all of the story. It appears that deafness is not inherited as a simple autosomal recessive (or dominant) defect, nor is it X-linked or mitochondrial DNA-linked inheritance. My suspicion is that two genes are involved: piebald and another gene that regulates how strongly the piebald gene is expressed, in other words, how strongly it impacts the dog. We know that the deafness occurs at 3-4 weeks after birth, when the blood supply to the cochlea (the stria vascularis) degenerates, and this degeneration appears to result from the absence of pigment cells (melanocytes) in the stria. We don't know the role of melanocytes here, but they appear essential. The piebald gene acts by suppressing melanocytes to give white. Here are my observations and the explanation I have developed from them. When the extreme piebald gene is weakly expressed, white does not entirely cover up the underlying coat color of black or liver, and a patch develops; these dogs are statistically less likely to become deaf (but this CAN happen - it is just unusual). When the extreme piebald gene is strongly expressed, melanocytes in the iris are suppressed, giving a blue eye, melanocytes behind the retina are suppressed, giving the red eye shine from the unmasked blood vessels, and melanocytes in the stria vascularis are suppressed, giving deafness. In the Scandinavian countries when they bred away from blue eyes they reduced deafness. Blue eyes are not allowed in the breed standard for the Dalmatian in Europe, Canada, and Mexico, but are allowed in the breed standard of the US. Deafness prevalence is much higher in the US (8% both ears, 22% one ear) than in the UK and Netherlands; the only difference I can identify to explain the deafness prevalence differences is eye color. Also, I don't advocate breeding FOR patches, but I think efforts to eliminate patches entirely will increase the likelihood of deafness. Thus, it should be possible, if we can identify the gene regulating piebald, to reduce (maybe even eliminate) deafness without changing the appearance of the breed. A US colleague and I have initiated a project to identify the gene defect responsible for pigment-associated deafness, starting with the Dalmatian but also including the English Setter and English Cocker spaniel. If successful, a blood test to identify carriers of the defect(s) will be developed, and breeders will have a means to select potential breeding mates without the defect. For those unaware of it, my web site on Deafness in Dogs & Cats (www.lsu.edu/guests/senate/public_html/deaf.htm) is a good source for information on deafness, hearing testing (BAER), available BAER sites, and other resources. Those interested in communicating with me can do so at strain@lsu.edu I hope this has been of some help. George M. Strain |
Last Updated on the 21st January 2001