BACKGROUND
the association between patterns of pigmentation and deafness in the dog has a long-documented history, with reports dating back over one hundred years.
long suspected of having a genetic basis, the search for loci with a pronounced influence in the expression of hearing loss in the dog has yet to be successful.
one study  <cit>  has discussed various candidate loci based upon research on deafness in the dog, the human, and the mouse, but to date no specific loci have been shown to influence deafness in the dog.
the interested reader is directed to a more comprehensive review of a molecular genetic approach to deafness in dogs, one outlining the application of comparative genomics  <cit> .
perhaps the most dramatic association between patterns of pigmentation and deafness in the dog can be found in the dalmatian.
in the dalmatian, iris colour is positively correlated with deafness  and the presence of a colour patch is negatively correlated with deafness   <cit> .
no studies in the dog to date have found a possible influence of a specific colour locus  associated with deafness  <cit> .
there is a review of several breeds known to have a high risk for pigment-associated sensorineural deafness  <cit> .
in addition to the dalmatian, the review reported evidence showing white bull terriers are more likely to be deaf than bull terriers with colour in their coats.
the jack russell terrier  was also examined, however with records on only  <dig> dogs it could only be noted that  <dig> of the  <dig> dogs had normal hearing, with  <dig> dogs being unilaterally deaf and  <dig> bilaterally deaf.
more observations would be needed in order to attempt to substantiate a genetic component to hearing loss in the jrt.
the breed standard for the jrt includes a requirement that the dog's body be more than 51% white with tan, black, or brown markings  <cit> .
the requirement that a jrt coat be 51% or more white creates variation within the breed ranging from dogs with a large portion of colour in their coats  to dogs that are mostly white  as shown in figure  <dig>  the jrt standard also describes three different types of coat texture: smooth, rough, and broken   <cit> .
examples of smooth texture and broken texture are shown in figure  <dig>  the jrt can also have blue eyes, but a jrt with blue eyes is rare compared to the incidence in other breeds .
to the author's knowledge, no previous study has examined the influence of coat colour or coat texture on deafness in the jrt.
the present study is intended to evaluate the heritability of deafness in the jrt, characterize the mode of inheritance, and evaluate the existence of a sex, coat colour, or coat texture influence on the expression of sensorineural deafness.
RESULTS
of the  <dig> dogs  with known auditory status,  <dig> dogs  had normal hearing,  <dig>  were unilaterally deaf and  <dig>  were bilaterally deaf .
figure  <dig> shows a subset pedigree of  <dig> dogs with known auditory status from the dataset.
there are several males with a great impact on this pedigree , including a few instances of inbreeding.
specifically, of the  <dig> total animals represented in the pedigree,  <dig> were inbred, with an average inbreeding coefficient of  <dig>  as calculated using the program mtgsam  <cit> .
a preliminary analysis of the deafness data without considering the potential contribution of inheritance  found none of the included fixed effects to be significant contributors to the incidence of deafness .
similar conclusions  could be drawn from a preliminary analysis of the three-category phenotype of deafness using a multinomial model in r <cit> .
a auditory status/coat texture/coat colour and sex.
b phenotypes of jack russell terriers from the assembled pedigree.
dogs with at least one unknown trait are not included in this table, but were included in the dataset.
a estimates are taken from a gibbs sample of  <dig>  values.
b gibbs sample convergence statistic; geweke .
c high density region.
d relative risk is computed for the probability of at least one deaf ear for white male versus tricolour male, white male versus tan male and white female versus white male, respectively.
also presented in table  <dig> is the influence of coat colour in the incidence of jrt deafness.
consistent for each measure of hearing loss, the difference between mostly white dogs and those with a tan colour pattern indicates that dogs with more white are more likely to be deaf.
this conclusion is justified by examination of the empirical 95% highest density region  generated by the  <dig>  gibbs samples.
specifically, the comparison of white and tan coloured dogs found that 95% of the gibbs sample estimates of this difference are between  <dig>  and  <dig>  , an interval that does not overlap with  <dig>  given that we scored the trait  <dig> for deaf and  <dig> for unaffected, larger values of the parameter θ  imply a greater risk for deafness.
only the contrast for white and tan coated dogs had an hdr that did not overlap  <dig>  accordingly, only selected contrasts are presented in table  <dig>  as mentioned previously, there was only one black and white jrt with deafness  in the pedigree, hence there was insufficient data to analyze this separate coat colour category and its possible influence on deafness.
however, the contrasting results observed between the mostly white dogs and those with a tan colour pattern are sufficient to conclude a colour influence on the expression of deafness in the jrt.
the contrast for coat texture had an hdr overlapping  <dig> .
note, the significant contribution of the tan coat colour stands in contrast to the result found when the genetic contribution to deafness was not considered.
a estimates are taken from a gibbs sample of  <dig>  values.
b gibbs sample convergence statistic; geweke .
c high density region.
d genetic correlation.
e phenotypic correlation
a estimates are taken from a gibbs sample of  <dig>  values.
b mendelian transmission parameter; the probability of transmitting an "a" allele.
for mendelian transmission these values are fixed as  <dig> , .
<dig> and  <dig>  for putative major genotypes aa, ab and bb, respectively.
non-mendelian transmission implies estimation of these values from the data.
c effective sample size.
d gibbs sample convergence statistic; geweke .
e highest density region
not presented is an analysis of the trichotomous definition of deafness because such a phenotype cannot be evaluated legitimately with the ibay software; the software is limited to the evaluation of binary and normally distributed phenotypes.
however, fitting the values of normal hearing, unilateral deafness and bilateral deafness as scores of  <dig>   <dig>   <dig>  respectively, did support the conclusion of a segregating major locus .
discussion
this is the first attempt, to our knowledge, to characterize a genetic component responsible for deafness in the jrt.
it is clear from the results presented in table  <dig> that deafness in the jrt is hereditary and is influenced by genetic information passed from parent to offspring.
as such, the heritability of deafness is of sufficient magnitude that attempts to select against it are potentially successful.
however, the heritability of deafness reported here for the jrt is lower than the estimates of the heritability of deafness in the dalmatian  <cit> , the only other breed with such estimates to the authors' knowledge.
while the prevalence of deafness in these respective breeds is also not the same  <cit> , as the jrt is less affected compared to dalmatians, differing estimates of the heritability of deafness between the breeds could be suggestive of dissimilar mechanisms resulting in a similar phenotype.
there are obvious issues  in comparisons between studies analyzing estimations of heritability within one breed.
those issues also apply in comparison of this study to any conducted utilizing another breed.
however, since this is the only study to date analyzing deafness in the jrt, comparisons to studies in the dalmatian are all that are available.
unlike the dalmatian, where the heritability of deafness has been estimated to be as high as  <dig>  for a trichotomous trait  <cit> , the highest heritability estimate in the jrt in this study of  <dig>  for a trichotomous trait is not indicative by itself of a single major locus exerting a large effect.
it has been demonstrated  <cit>  that major loci tend to increase the heritability of a trait in a given population and a value greater than  <dig>  is comparatively large for many polygenic traits.
because the estimate in the jrt is markedly below  <dig> , other loci may be exerting an effect on any major locus responsible for deafness in the jrt.
to further support this, table  <dig> presents the results of the complex segregation analysis whereby the model of a single locus with a large effect on hearing loss is not supported.
though we can conclude that deafness is heritable from the results in table  <dig>  the exact genetic mechanism that leads to expression of this disease cannot be stated with certainty based on the results in table  <dig>  a manual review of the pattern of inheritance did not support a model of a simple autosomal mendelian locus.
for example, the majority of the affected progeny were the result of matings of two unaffected parents, eliminating models of a single dominant deafness allele.
discarding a model of a single recessive autosomal allele is not possible with the pedigree, because there were no matings of two bilaterally deaf dogs; nor was there any mating of two unilaterally deaf dogs.
once again, the reader is reminded that our evaluation of a major locus may be influenced by possible ascertainment bias, our sample of dogs being built upon affected animals.
regrettably, there is no simple means by which this potential effect can be eliminated from the present analysis.
what remains is the need for a cautious interpretation of the results of the several analyses.
also presented in table  <dig> is the influence of coat colour and coat texture in the incidence of jrt deafness.
consistent for each measure of hearing loss, there is a difference between mostly white dogs and those with a tan colour pattern.
this indicates that mostly white dogs are more likely to be deaf, supporting that deafness in the jrt ; there is a pigmentation association with deafness.
the breed standard for the jrt mandates that the body be at least 51% white  <cit> .
unfortunately, no jrts with blue eyes were available with this pedigree to evaluate a possible association between eye colour and deafness as has been observed in the dalmatian.
however, as the multiple trait analysis presented in table  <dig> reveals, the association between coat colour and deafness appears to be less pronounced in the jrt than in the dalmatian.
though the mean estimate of the genetic correlation  is strong and positive , the 95% hdr suggests that estimate to be quite imprecise.
perhaps a larger sample of dogs, or a better means of quantifying coat colour would reveal a more precise relationship between hearing loss and pigmentation.
although deafness in the jrt is clearly inherited, the evidence for the presence of a single major gene affecting the disorder is not persuasive with the data from this pedigree.
in a review of complex segregation analysis  <cit> , it was suggested to exercise caution in the interpretation of complex segregation analysis until several sets of data had confirmed or rejected the presence of a mendelian locus.
further studies will be valuable in this context.
CONCLUSIONS
the objectives of this study were to evaluate the heritability of deafness and the existence of a sex, coat colour, and/or coat texture influence on the expression of deafness in the jrt as well as characterize the mode of inheritance.
the heritability of deafness in the jrt was found to be  <dig>  and  <dig>  considering deafness to be dichotomous and trichotomous, respectively.
there appears to be an influence of coat colour on the expression of deafness as a difference was observed between mostly white jrts and those with a tan colour pattern, indicating that white jrts are more likely to be deaf.
in an attempt to characterize the mode of inheritance of deafness in the jrt, a model of a single locus with a large effect on hearing loss is not supported with this data.
further study is needed to determine if a single locus may be influencing deafness in the jrt.
while the absence of a clear mode of inheritance complicates genetic dissection of deafness in the jrt, the assembling of this pedigree provides a tool for eventually defining the genetic bases of this disorder.
initially, one study  <cit>  reported a heritability estimate of  <dig>  in californian dalmatians, a value comparable to the estimate presented here in jrts.
however, a subsequent study  <cit>  included more records of californian dalmatians in a larger data set and reported a higher heritability estimate of  <dig> , a value comparable to the estimates presented by other studies involving dalmatians  <cit> .
inclusion of more individuals related to those in this pedigree may provide the opportunity for a more in-depth analysis of the heritability of deafness in jrts.
also, collection of additional unilaterally or bilaterally deaf jrts with black and white coat, as well as collection of normal and affected jrts with blue eyes, will further elucidate the influence of pigmentation on the expression of deafness in this breed.

 the association between patterns of pigmentation and deafness in the dog has a long-documented history, with reports dating back over one hundred years. long suspected of having a genetic basis, the search for loci with a pronounced influence in the expression of hearing loss in the dog has yet to be successful. one study has discussed various candidate loci based upon research on deafness in the dog, the human, and the mouse, but to date no specific loci have been shown to influence deafness in the dog. the interested reader is directed to a more comprehensive review of a molecular genetic approach to deafness in dogs, one outlining the application of comparative genomics. perhaps the most dramatic association between patterns of pigmentation and deafness in the dog can be found in the dalmatian. in the dalmatian, iris colour is positively correlated with deafness and the presence of a colour patch is negatively correlated with deafness. no studies in the dog to date have found a possible influence of a specific colour locus associated with deafness. there is a review of several breeds known to have a high risk for pigment-associated sensorineural deafness. in addition to the dalmatian, the review reported evidence showing white bull terriers are more likely to be deaf than bull terriers with colour in their coats. the jack russell terrier was also examined, however with records on only dogs it could only be noted that of the dogs had normal hearing, with dogs being unilaterally deaf and bilaterally deaf. more observations would be needed in order to attempt to substantiate a genetic component to hearing loss in the jrt. the breed standard for the jrt includes a requirement that the dog's body be more than 51% white with tan, black, or brown markings. the requirement that a jrt coat be 51% or more white creates variation within the breed ranging from dogs with a large portion of colour in their coats to dogs that are mostly white as shown in figure the jrt standard also describes three different types of coat texture: smooth, rough, and broken. examples of smooth texture and broken texture are shown in figure the jrt can also have blue eyes, but a jrt with blue eyes is rare compared to the incidence in other breeds. to the author's knowledge, no previous study has examined the influence of coat colour or coat texture on deafness in the jrt. the present study is intended to evaluate the heritability of deafness in the jrt, characterize the mode of inheritance, and evaluate the existence of a sex, coat colour, or coat texture influence on the expression of sensorineural deafness.  of the dogs with known auditory status, dogs had normal hearing, were unilaterally deaf and were bilaterally deaf. figure shows a subset pedigree of dogs with known auditory status from the dataset. there are several males with a great impact on this pedigree, including a few instances of inbreeding. specifically, of the total animals represented in the pedigree, were inbred, with an average inbreeding coefficient of as calculated using the program mtgsam. a preliminary analysis of the deafness data without considering the potential contribution of inheritance found none of the included fixed effects to be significant contributors to the incidence of deafness. similar conclusions could be drawn from a preliminary analysis of the three-category phenotype of deafness using a multinomial model in r. a auditory status/coat texture/coat colour and sex. b phenotypes of jack russell terriers from the assembled pedigree. dogs with at least one unknown trait are not included in this table, but were included in the dataset. a estimates are taken from a gibbs sample of values. b gibbs sample convergence statistic; geweke. c high density region. d relative risk is computed for the probability of at least one deaf ear for white male versus tricolour male, white male versus tan male and white female versus white male, respectively. also presented in table is the influence of coat colour in the incidence of jrt deafness. consistent for each measure of hearing loss, the difference between mostly white dogs and those with a tan colour pattern indicates that dogs with more white are more likely to be deaf. this conclusion is justified by examination of the empirical 95% highest density region generated by the gibbs samples. specifically, the comparison of white and tan coloured dogs found that 95% of the gibbs sample estimates of this difference are between and, an interval that does not overlap with given that we scored the trait for deaf and for unaffected, larger values of the parameter θ imply a greater risk for deafness. only the contrast for white and tan coated dogs had an hdr that did not overlap accordingly, only selected contrasts are presented in table as mentioned previously, there was only one black and white jrt with deafness in the pedigree, hence there was insufficient data to analyze this separate coat colour category and its possible influence on deafness. however, the contrasting results observed between the mostly white dogs and those with a tan colour pattern are sufficient to conclude a colour influence on the expression of deafness in the jrt. the contrast for coat texture had an hdr overlapping. note, the significant contribution of the tan coat colour stands in contrast to the result found when the genetic contribution to deafness was not considered. a estimates are taken from a gibbs sample of values. b gibbs sample convergence statistic; geweke. c high density region. d genetic correlation. e phenotypic correlation a estimates are taken from a gibbs sample of values. b mendelian transmission parameter; the probability of transmitting an "a" allele. for mendelian transmission these values are fixed as,. and for putative major genotypes aa, ab and bb, respectively. non-mendelian transmission implies estimation of these values from the data. c effective sample size. d gibbs sample convergence statistic; geweke. e highest density region not presented is an analysis of the trichotomous definition of deafness because such a phenotype cannot be evaluated legitimately with the ibay software; the software is limited to the evaluation of binary and normally distributed phenotypes. however, fitting the values of normal hearing, unilateral deafness and bilateral deafness as scores of respectively, did support the conclusion of a segregating major locus. discussion this is the first attempt, to our knowledge, to characterize a genetic component responsible for deafness in the jrt. it is clear from the results presented in table that deafness in the jrt is hereditary and is influenced by genetic information passed from parent to offspring. as such, the heritability of deafness is of sufficient magnitude that attempts to select against it are potentially successful. however, the heritability of deafness reported here for the jrt is lower than the estimates of the heritability of deafness in the dalmatian, the only other breed with such estimates to the authors' knowledge. while the prevalence of deafness in these respective breeds is also not the same, as the jrt is less affected compared to dalmatians, differing estimates of the heritability of deafness between the breeds could be suggestive of dissimilar mechanisms resulting in a similar phenotype. there are obvious issues in comparisons between studies analyzing estimations of heritability within one breed. those issues also apply in comparison of this study to any conducted utilizing another breed. however, since this is the only study to date analyzing deafness in the jrt, comparisons to studies in the dalmatian are all that are available. unlike the dalmatian, where the heritability of deafness has been estimated to be as high as for a trichotomous trait, the highest heritability estimate in the jrt in this study of for a trichotomous trait is not indicative by itself of a single major locus exerting a large effect. it has been demonstrated that major loci tend to increase the heritability of a trait in a given population and a value greater than is comparatively large for many polygenic traits. because the estimate in the jrt is markedly below, other loci may be exerting an effect on any major locus responsible for deafness in the jrt. to further support this, table presents the results of the complex segregation analysis whereby the model of a single locus with a large effect on hearing loss is not supported. though we can conclude that deafness is heritable from the results in table the exact genetic mechanism that leads to expression of this disease cannot be stated with certainty based on the results in table a manual review of the pattern of inheritance did not support a model of a simple autosomal mendelian locus. for example, the majority of the affected progeny were the result of matings of two unaffected parents, eliminating models of a single dominant deafness allele. discarding a model of a single recessive autosomal allele is not possible with the pedigree, because there were no matings of two bilaterally deaf dogs; nor was there any mating of two unilaterally deaf dogs. once again, the reader is reminded that our evaluation of a major locus may be influenced by possible ascertainment bias, our sample of dogs being built upon affected animals. regrettably, there is no simple means by which this potential effect can be eliminated from the present analysis. what remains is the need for a cautious interpretation of the results of the several analyses. also presented in table is the influence of coat colour and coat texture in the incidence of jrt deafness. consistent for each measure of hearing loss, there is a difference between mostly white dogs and those with a tan colour pattern. this indicates that mostly white dogs are more likely to be deaf, supporting that deafness in the jrt; there is a pigmentation association with deafness. the breed standard for the jrt mandates that the body be at least 51% white. unfortunately, no jrts with blue eyes were available with this pedigree to evaluate a possible association between eye colour and deafness as has been observed in the dalmatian. however, as the multiple trait analysis presented in table reveals, the association between coat colour and deafness appears to be less pronounced in the jrt than in the dalmatian. though the mean estimate of the genetic correlation is strong and positive, the 95% hdr suggests that estimate to be quite imprecise. perhaps a larger sample of dogs, or a better means of quantifying coat colour would reveal a more precise relationship between hearing loss and pigmentation. although deafness in the jrt is clearly inherited, the evidence for the presence of a single major gene affecting the disorder is not persuasive with the data from this pedigree. in a review of complex segregation analysis, it was suggested to exercise caution in the interpretation of complex segregation analysis until several sets of data had confirmed or rejected the presence of a mendelian locus. further studies will be valuable in this context.  the objectives of this study were to evaluate the heritability of deafness and the existence of a sex, coat colour, and/or coat texture influence on the expression of deafness in the jrt as well as characterize the mode of inheritance. the heritability of deafness in the jrt was found to be and considering deafness to be dichotomous and trichotomous, respectively. there appears to be an influence of coat colour on the expression of deafness as a difference was observed between mostly white jrts and those with a tan colour pattern, indicating that white jrts are more likely to be deaf. in an attempt to characterize the mode of inheritance of deafness in the jrt, a model of a single locus with a large effect on hearing loss is not supported with this data. further study is needed to determine if a single locus may be influencing deafness in the jrt. while the absence of a clear mode of inheritance complicates genetic dissection of deafness in the jrt, the assembling of this pedigree provides a tool for eventually defining the genetic bases of this disorder. initially, one study reported a heritability estimate of in californian dalmatians, a value comparable to the estimate presented here in jrts. however, a subsequent study included more records of californian dalmatians in a larger data set and reported a higher heritability estimate of, a value comparable to the estimates presented by other studies involving dalmatians. inclusion of more individuals related to those in this pedigree may provide the opportunity for a more in-depth analysis of the heritability of deafness in jrts. also, collection of additional unilaterally or bilaterally deaf jrts with black and white coat, as well as collection of normal and affected jrts with blue eyes, will further elucidate the influence of pigmentation on the expression of deafness in this breed. 
no studies in the dog to date have found a possible influence of a specific colour locus associated with deafness. the present study is intended to evaluate the heritability of deafness in the jack russell terrier, characterize the mode of inheritance, and evaluate the existence of a sex, coat colour, or coat texture influence on the expression of sensorineural deafness. complex segregation analysis revealed a model of a single locus with a large effect on the binary measure of hearing loss is not supported. while the absence of a clear mode of inheritance complicates genetic dissection of deafness in the jrt, the assembling of this pedigree provides a tool for eventually defining the genetic bases of this disorder. this is the first attempt, to our knowledge, to characterize a genetic component responsible for deafness in the jrt. long suspected of having a genetic basis, the search for loci with a pronounced influence in the expression of hearing loss in the dog has yet to be successful. the influence of coat colour in the incidence of jrt deafness was statistically significant, indicating that dogs with more white are more likely to be deaf. the influence of sex or coat texture was not statistically significant in the incidence of jrt deafness. in an attempt to characterize the mode of inheritance of deafness in the jrt, a model of a single locus with a large effect on hearing loss is not supported with this data. there appears to be an influence of coat colour on the expression of deafness.
 the association between patterns of pigmentation and deafness in the dog has a long-documented history, with reports dating back over one hundred years. long suspected of having a genetic basis, the search for loci with a pronounced influence in the expression of hearing loss in the dog has yet to be successful. no studies in the dog to date have found a possible influence of a specific colour locus associated with deafness. the present study is intended to evaluate the heritability of deafness in the jack russell terrier, characterize the mode of inheritance, and evaluate the existence of a sex, coat colour, or coat texture influence on the expression of sensorineural deafness.  the estimation of heritability of deafness in the jrt was when deafness was considered a binary trait and when deafness was considered a three-category. the influence of coat colour in the incidence of jrt deafness was statistically significant, indicating that dogs with more white are more likely to be deaf. the influence of sex or coat texture was not statistically significant in the incidence of jrt deafness. complex segregation analysis revealed a model of a single locus with a large effect on the binary measure of hearing loss is not supported.  this is the first attempt, to our knowledge, to characterize a genetic component responsible for deafness in the jrt. the heritability of deafness in the jrt was found to be and considering deafness to be a two-category or three-category trait, respectively. there appears to be an influence of coat colour on the expression of deafness. in an attempt to characterize the mode of inheritance of deafness in the jrt, a model of a single locus with a large effect on hearing loss is not supported with this data. further study is needed to determine if a single locus may be influencing deafness in the jrt. while the absence of a clear mode of inheritance complicates genetic dissection of deafness in the jrt, the assembling of this pedigree provides a tool for eventually defining the genetic bases of this disorder. 