Coat colour inheritance in horses

The colours of the horses have long been a subject of interest to owners and breeders of horses as well as to scientists. Though, the colour of horses has little to do with its performance, it is a primary means of identification and also the first indicator of questionable parentage. Probably the ancestral colour of the horse was a black-based pattern that provided camouflage protection against predators. Horse colours are mostly controlled by genes at 12 different loci. The three basic colours of horses are black, bay and chestnut. The genetic control of the basic colours of horses resides at two genetic loci, namely Extension (E) and Agouti (A) loci. Among the basic colours bay is dominant to black and both are epistatic to chestnut. Dilution of basic colours of horses as a result of four colour dilution genes such as cream dilution, dun, silver dapple and champagne resulted in extensive array of possible colours of horses. The most widespread and familiar of the horse colour dilution gene is the one that produces the golden body colour and are called as palomino or buckskin based on the colour of the points. The grey coat colour is due to the presence of dominant gene (G) at the grey locus. Grey is epistatic to all coat colour genes except white and a grey horse must have at least one grey parent. Roan is due to a dominant gene (Rn) at roan locus and this combines with any base colour to produce the various shades of roan pattern. White coat is due to a single dominant gene (W) and it is epistatic to the genes controlling all other colours. White marking in the face and legs are due to genetic and non-genetic factors. Several genes are involved in producing white markings. During recent years, comparative genomics and whole genome scanning have been used to develop DNA tests for different variety of horse colours. Molecular genetic studies on coat colour in horses helped in identification of the genes and mutation responsible for coat colour variants. In future, this will be applied to breeding programmes to reduce the incidence of diseases and to increase the efficiency of race horse population.     

Morphological and genetic diversity of Pura Raza Español horse with regard to the coat colour

Gene mutations influencing melanocytes also impact on physiological and behavioural functions. In this study, we investigated their association with four different coat colours in the Pura Raza Español (PRE) horse using morphological traits and molecular datasets. Four different subpopulations were identified according to individual coat colour: grey, bay, chestnut and black. Coat colour significantly associated with morphological measurements. Observed and expected heterozygosity values were low in grey compared with the other three subpopulations, suggesting the presence of unique ancestral alleles probably arisen by genetic drift and selection mechanism effects. Nei's distance demonstrated a clear division among subpopulations, the grey being the most divergent group. Gene flow estimates were similar, showing the lowest values in grey. Divergence times among subpopulations assessed with the average square distance suggested that grey was the original PRE population which diverged from bay, chestnut and black. Our results also demonstrated a clear morphological differentiation according to coat colour. The close genetic structure of bay and chestnut PRE subpopulations and the clear differences in most morphological traits of grey and chestnut PRE mares would suggest the pleiotropic effect of genomic regions determining coat colour in horses. However, further analysis including genomic information would be necessary to elucidate the mechanisms involved.     

Tracing the origin of 'blue Weimaraner' dogs by molecular genetics

Weimaraner dogs are defined by light brown coat colour termed grey including several shadings ranging from silver and deer to mouse grey. In contrast, the so-called blue Weimaraners (BW) with lightened black-pigmented coat have been proposed to represent spontaneous revertants in the Weimaraner breed. In order to investigate the genetic determinants of the characteristic grey coat colour versus those of BW, known variation in coat colour genes including TYRP1 and MLPH were analysed in a number of grey and blue dogs. Variations at the B locus cause grey coat colour in Weimaraners via two non-functional TYRP1 copies (bb) including the b(s), b(d) and b(c) alleles. In all BW, at least one functional TYRP1 allele (Bb or BB genotype) was identified. Defined microsatellite alleles in TYRP1 intron 4 are linked to this functional B allele in BW. These alleles were also detected in various other dog breeds, but not in grey Weimaraners. The combination of a dominant trait for blue versus grey together with a specific TYRP1 haplotype in BW suggests that blue coat colour is not the result of spontaneous (back-) mutation in grey Weimaraners. This inference is even emphasized by the presence of a unique Y-chomosomal haplotype in a male offspring of the supposed ancestor of the BW population which - according to pedigree information - carries a copy of the original Y chromosome. Thus, molecular genetic analyses of coat colours combined with Y-chromosomal haplotypes allow tracing the origin of atypical dogs in respective canine populations.     

Coat colour genes in diversity studies

In this paper we describe the use of polymorphic genes affecting coat colour as a tool in diversity studies of domestic animals. Although phenotypic data has been the main criteria for establishing different breeds, calculation of genetic distances between breeds is normally performed using noncoding microsatellite markers. As anticipated, MC1‐R (melanocyte stimulating hormone receptor) allele frequencies vary greatly between cattle breeds expressing different coat colours. In multicoloured breeds, like Icelandic cattle, a high frequency of the E⁺ allele appears to be essential for colour variation. Whereas black breeds have a high frequency of the dominant acting allele E^D, entirely red breeds have no E^D. Animals being homozygous for the defective allele e occurred frequently in some cattle breeds, indicating that the MC1‐R does not have crucial impact on animal physiology other than coat colour. The E⁺ and e alleles were observed in the closely related river buffalo as well. None of the breeds included in this study express the roan phenotype. Consequently, they were monomorphic at the MGF locus. As for the MC1‐R locus, a correlation to colour pattern was observed for two c‐kit alleles as well, confirming that selection of specific phenotypes strongly affect the allelic variation of underlying loci. Information on genes affecting the phenotype is therefore well suited for describing different breeds of livestock and, consequently, a practical tool in breed conservation.     

Variation of gene frequencies in ASIP, MC1R and GREY loci in Thoroughbred horses

The objective of the study has been to verify the hypothesis that the coat colour is regarded in the selection of Polish Thoroughbred horse population. Formally, the colour is not a selection criterion in this breed selected mainly for speed. The material consisted of twelve groups of foals registered in successive volumes of the Stud Book (11,688 foals, in total) and their parents selected to the breeding stud. The frequency of alleles in ASIP, MC1R and GREY loci controlling the coat colours was estimated from the recessive phenotype frequency square in the groups of foals. The inflow of foreign genes was limited and the population great, hence the migration effect was very low. The drift and Wahlund effect hardly influenced the genetic structure in the groups which enabled to analyze the population not divided. The total offspring frequency of recessive a, e and g alleles amounted to 0.1552, 0.4877 and 0.9773, respectively. Accuracy of the assessment of the a and e frequency was confirmed on the basis of test matings. The a, e and g alleles were more frequent in dams than in sires and the a alleles occurred more often in fillies than in colts. The frequency of a and e alleles was higher in the offspring than in the parents. The genotype distribution in the offspring differed from the expected one, assessed from the gamete frequency in sires and in dams. Fewer bay foals were born than anticipated. All the results show that the coat colour is not entirely disregarded in the breeding of Thoroughbred horses. The dominant A and E alleles producing the colour are preferred in the selection, particularly in the sires. This leads to some alterations in the phenotypic structure of the population. On the other hand, the horses are mated randomly, irrespective of the coat colour.     

Anaplastic malignant melanoma of the tail in non‐grey horses

Information regarding signalment, clinical findings, treatment and outcome of 5 previously reported cases of anaplastic malignant melanoma of the tail in non‐grey horses and of 5 additional cases are summarised. Age was recorded for 9 horses and mean age was 16 years, range 8–23 years. Gender was recorded for 8 horses and 6 of these 8 horses were male horses over 14 years of age. The most common coat colour was bay (6 horses). Other coat colours were palomino (one horse), chestnut (one horse) and black (one horse); coat colour of one non‐grey horse was not specified. Follow‐up information was available for 9 horses and only one horse, a palomino, survived more than 10 months following diagnosis and tail amputation. Surgical excision, including tail amputation and medical therapy with oral cimetidine, was not effective in non‐grey, non‐palomino horses. Tumour recurred on tail tissue remaining after amputation in 2 horses, widespread metastases were documented in 4 cases and metastasis was suspected at the time of death or euthanasia in 3 cases, including one case with amputation site regrowth. No subjective histopathological differences were detected in the palomino horse that survived as compared to horses of other coat colours. Findings suggest that anaplastic malignant melanoma of the tail in non‐grey horses is most often a very aggressive neoplasm, but that there are rare exceptions.     

Patterns of scintigraphic uptake in the fetlock joint of Thoroughbred racehorses and the effect of increased radiopharmaceutical uptake in the distal metacarpal/tarsal condyle on performance

Reasons for performing study: The effect of palmar osteochondral disease lesions on performance of Thoroughbred (TB) racehorses is unclear. There is a need to describe patterns of radiopharmaceutical uptake (IRU) in fetlock joints of TB racehorses and to evaluate post scintigraphy performance. Hypotheses: IRU in the metacarpal (MC) and metatarsal (MT) condyles is more common than IRU in the parasagittal grooves and is associated with poorer post diagnosis performance than controls. Methods: Location of IRU within the fetlock region was identified and graded subjectively in TB racehorses. Performance variables were determined from race records for horses with moderate/marked MC/MT condylar IRU (cases), other horses undergoing scintigraphy (scintigraphy controls) and age/sex matched controls from the last race in which a case participated (controls). Statistical analyses included quantile regression, Fisher's exact test, Kaplan‐Meier survival curves, log‐rank test and Cox regression. Results: Metacarpal/MT condylar IRU was identified in 103/220 horses with only 3/220 having parasagittal IRU. Moderate/marked IRU was identified in the MC and MT condylar regions in 62 and 39 horses, respectively, with palmaromedial and plantarolateral IRU most common. Fore‐ and hindlimb cases had fewer starts, reduced earnings (P<0.001) and reduced earnings/start than controls respectively. Cases were more likely to return to racing later than controls following a rest period. Cases were older than scintigraphy controls. Conclusions: In TB racehorses presenting for lameness MC/MT condylar IRU is the most common abnormality identified within the fetlock joint. Racehorses with moderate/marked condylar IRU have a shortened racing career, are less successful than age/sex matched controls and are older than other racehorses presented for scintigraphy. Potential relevance: Overload of the MC/MT condyles is a common and significant problem in TB racehorses that is readily identified with scintigraphy. Scintigraphy of horses that are lame or performing poorly is less useful for screening for potential condylar fractures.     

猪的毛色遗传研究进展

猪的毛色遗传极其复杂,至少受7个位点基因控制.现综述猪毛色的类型、猪毛色的遗传以及猪毛色基因的分子生物学基础研究进展.对目前我国养猪业中出现的一些毛色异常现象进行了分析.     

The genetics of colour in fat-tailed sheep: a review

Fat-tailed sheep come in various colours—most are either brown (tan) or black. In some, most of the body is white with the tan or black colour restricted to the front portion of the body or to just around the eyes, muzzle and parts of the legs. The Karakul breed is important for the production of lamb skins of various colours for the fashion industry. As well as the black and tan colours there are Karakuls bred for grey or roan shades, a white colour or one of the numerous Sur shades. In the Sur shades, the base of the birthcoat fibre is one of a number of dark shades and the tip a lighter or white shade. All these colours and many others are the result of the interaction of various genes that determine the specifics of the coat colour of the sheep. A number of sets of nomenclature and symbols have been used to represent the various loci and their alleles that are involved. In the 1980s and 1990s, a standardised set, based closely on those of the mouse and other species was developed. Using this as the framework, the alleles of the Extension, Agouti, Brown, Spotting, Pigmented Head and Roan loci are described using fat-tailed sheep (mainly Damara, Karakul and Persian) as examples. Further discussion includes other types of “white markings,” the Ticking locus and the Sur loci.     

Phenotypic and genetic parameter estimates for racing traits of Arabian horses in Turkey

The racing records for Arabian horses used in the study were obtained from the Turkish Jockey Club. The traits used in the study were racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start. Genetic parameters were estimated by the restricted maximum likelihood (REML) procedure using the DFREML program. The effects of age, sex and origin of horse were significant for each trait. The effect of year was significant on time and earning traits, but not rank. The effect of month on time traits was also significant. Heritability estimates of the entire data set were 0.280, 0.281, 0.069, 0.139, 0.174, 0.152 and 0.171 for racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start respectively. Estimates of repeatability varied from 0.349 to 0.500 for racing time, from 0.430 to 0.524 for best racing time and from 0.129 to 0.171 for rank depending on the data set used in the analyses. Best racing time was the most appropriate trait for selection in this study, as this might lead to genetic improvement than other traits.     

A longitudinal study of racing thoroughbreds: performance during the first years of racing

OBJECTIVES: To develop a profile of the racing careers of Thoroughbred horses in south-eastern Queensland, and to examine factors that affect racing during the first years of racing. DESIGN AND PROCEDURE: A longitudinal study using the racing records of a defined cohort of Thoroughbred horses that were born on or within 12 months following 1 August 1991. Data collection commenced in 1996 and will continue until all horses cease racing. In this paper two measures of performance were examined: race earnings during the first year of racing and cumulative proportion of horses still racing up to 2 years after their first start. Univariable and multivariable methods were used to describe and identify factors associated with the performance of these horses. RESULTS: By 31 July 1997, 1804 horses were enrolled in the study including 916, 701, 152 and 35 horses that first raced at 2, 3, 4 and 5 years of age, respectively. During their first year of racing, half the horses earned no more than A$450 from race earnings, and 710 (39.4%) horses earned no money at all. In comparison to poorly performing horses, well-performing horses were more likely to be male, to have started as 2-year-olds and to have had more starts during this year. Of the horses that first started as 2 and 3-year-olds, only 71 and 46% continued racing for at least 1 and 2 years after their first start, respectively. Length of racing life was associated with performance during the first year of racing (number of starts and average earnings per race), and with sex, date of birth and age at first start. CONCLUSIONS: The study confirmed a high wastage among racing Thoroughbreds. As expected, premature retirement from racing was linked to poor performance. During the first year of racing, the race earnings of an estimated 1567 (86.9%) horses were insufficient to cover training costs. The 2-year-old racing cohort outperformed the older racing cohorts in each of the performance measures under investigation. Interpretation of this result, and the long-term effects of 2-year-old racing, was limited by the problem of confounding.     

黑色素皮质激素受体1基因248位点多态性与中国马品种毛色的相关性分析

在欧洲马品种中,黑色素皮质激素受体1(melanocortin receptor 1,MC1R)基因第248位碱基有C/T多态性,纯合的T248位点决定欧洲马的栗毛色。针对MC1R基因的248位点设计了2对特异性引物,采用等位基因特异性PCR技术,研究3个中国马品种MC1R基因型与栗色毛之间的关系。经扩增获得两种DNA片段,克隆测序后证明,扩增片段确为MC1R基因,两种DNA片段序列在基因的248位点的确呈现C/T多态性,但检测126份贵州矮马、西南马和新疆伊犁马血液样本,全部为杂合基因型(Ee),其中包括栗毛、黑毛、骝毛3种单毛色及3种复毛色。这些研究结果提示,MC1R基因中248位点的多态性与国内3个马品种的栗色毛之间没有直接的相关性。     

Altered expression of melanocortin-1 receptor (MC1R) in a yellow-coloured wild raccoon dog (Nyctereutes procyonoides)

Background – The melanocortin 1 receptor (MC1R) gene plays a key role in determining coat colour in mammals by controlling the proportion of eumelanin and pheomelanin granules. Wild raccoon dogs have a mixed coat colour, with black to brown and grey hairs. Hypothesis/Objectives – The study was performed to identify the cause of the variant yellow coat colour in a wild raccoon dog. Animals – A wild raccoon dog that showed coat colour change to yellow and four wild‐type raccoon dogs that showed normal coat colour were included. Methods – To identify the cause of the variant yellow coat colour, we examined the sequence of the MC1R gene and its expression at the mRNA and protein levels. Results – The coding region of the MC1R gene of this raccoon dog comprised 954 bp, the same as for wild‐type raccoon dogs and domestic dogs. By comparing the gene with that in the wild‐type raccoon dog, a 2 bp deletion was detected in the 5′‐untranslated region, positioned 152 bp upstream of the start codon. However, there was no significant difference in the mRNA expression level. The yellow raccoon dog revealed a significantly decreased MC1R protein level compared with the wild‐type raccoon dogs, indicating an increase in pheomelanin synthesis. Conclusions and clinical importance – These results suggest that the variant coat colour in the yellow raccoon dog was associated with decreased MC1R function.     

Development of a method for simultaneously genotyping multiple horse coat colour loci and genetic investigation of basic colour variation in Thoroughbred and Misaki horses in Japan

In order to develop a genotyping method that can be used in the registration procedure for Thoroughbreds, we developed a method for simultaneously genotyping multiple coat colour genes on the basis of single nucleotide polymorphism typing by using the SNaPshot^TM technique. This method enabled precise and reasonable detection of causal mutations; it was effective for genotyping of MC1R, ASIP, and SLC45A2 at the Extension (E), Agouti (A), Cream dilution (C) loci, and the possibility of identification of rare variants of MC1R, EDNRB and KIT at the E, Overo (O) and Sabino 1 (SB1) loci, respectively, was also indicated. It was considered that this genotyping method would provide information not only for the registration of Thoroughbreds but also for the preservation of phenotypic characters, such as coat colour, of endangered Misaki native horses in Japan. Therefore, genetic variations at the five coat colour loci were investigated in 1111 Thoroughbred and 99 Misaki native horses. Allele frequencies at the polymorphic E and A loci were estimated, and the proportions of basic coat colours that could be expected in the Thoroughbred population were bay, 0.662; black, 0.070; chestnut, 0.268. In the Misaki population, they were bay, 0.792; black, 0.129; chestnut, 0.080. The data presented were the first of its kind on genetic coat colour variation, and will be important with regard to the registration of Thoroughbreds and the management of Misaki horses.     

Surgical treatment of dorsal cortical fractures of the third metacarpal bone in thoroughbred racehorses: 53 cases (1985-1989).

Between January 1985 and May 1989, 53 Thoroughbred horses (mean age 3.2 years) were surgically treated for dorsal cortical fractures of the third metacarpal bone (MC III). All horses were treated with cortical drilling through the fracture line (osteostixis). Diagnosis of the fractures was confirmed by xeroradiography. Lifetime racing records were obtained for all horses. Forty-seven horses returned to racing after surgery (89%). The mean time between surgery and the first race was 6.8 months. Horses had a mean of 10.9 starts before surgery and 16.1 starts after surgery. The mean earnings per start before surgery was $6,459 and after surgery was $5,685. Of the 47 horses that raced after surgery, 70% raced at the same class or improved. Complications related to surgery were seen in 10 horses. Two horses had a second fracture of MC III at the same site, and were again treated by osteostixis, after which both horses returned to competition. Fractured drill bits were left in the MC III of 4 horses. One of these horses had catastrophic failure of MC III. Two horses developed subcutaneous infections and 2 horses had catastrophic failure of MC III in the surgically treated limb. Osteostixis appears to be an effective treatment for returning horses affected with dorsal cortical fractures to racing.     

Comparison of two different statistical models considering individual races or racetracks for evaluation of German trotters

Two different statistical models considering racetrack or individual race as fixed effect were compared, regarding genetic parameters and by using cross validation. Data for variance component estimation consisted of 48,942 performance observations from 4249 trotters. Variance components for the traits square root of rank at finish, racing time per km, and log of earnings per race were estimated by REML using two multiple trait animal models involving different racetracks or individual races. When including each individual race instead of racetracks in the statistical model, heritabilities increased from 0.05 to 0.07, 0.19 to 0.23, and 0.08 to 0.09 for square root of rank at finish, racing time per km, and log of earnings per race, respectively. Genetic and phenotypic correlations among traits increased also after consideration of individual races. Square root of rank at finish, as well as racing time per km and log of earnings per race, was highly genetically correlated with −0.99 and −0.88. The two statistical models were compared on the basis of their predictive ability by using cross validation. Data for these analyses consisted of 706,082 observations from 21,363 trotters. Randomly eliminated performance observations were predicted by cumulation of fixed and random effects obtained from estimation of breeding values for both models. Estimates for racing time showed lower bias and mean square error (MSE) when considering individual races instead of racetracks. Also, the correlation between predicted and true phenotypic value increased from 0.85 to 0.92. Estimates for square root of rank at finish were unbiased, but with a higher MSE when considering individual race effect. A similar high bias and MSE with both models were obtained for log of earnings. In order to avoid bias in estimation of genetic parameters and breeding values for racing time and square root of rank at finish, inclusion of each individual race in the statistical model was recommended.     

A clinical evaluation of a headless, titanium, variable-pitched, tapered, compression screw for repair of nondisplaced lateral condylar fractures in thoroughbred racehorses

OBJECTIVE: To report clinical evaluation of headless compression screws for repair of metacarpal/metatarsal (MC/MT3) condylar fractures in horses. STUDY DESIGN: Retrospective case study. SAMPLE POPULATION: Racing Thoroughbreds (n=16) with nondisplaced lateral condylar fractures of MC/MT 3. METHODS: Medical records (1999-2004) of horses with nondisplaced longitudinal fractures of the lateral condyle of MC/MT3 were reviewed. Pre-operative variables retrieved were: patient age, gender, limb involvement, injury occurrence, fracture length, and width, evidence of palmar comminution and degenerative joint disease, number of pre-injury starts, and pre-injury earnings. Post-operative variables retrieved were: surgical complications, surgical time, number of race starts, and post-operative earnings. RESULTS: MC3 (n=11) and MT3 (5) nondisplaced longitudinal fractures of the lateral condyle were repaired with Acutrak Equine (AE) screws. Left front limb fractures were most common (8) followed by left hind (5) and right front (3). Nine fractures occurred during training and 7 during racing; 4 fractures had palmar comminution. No surgical complications occurred. Of 15 horses that returned to training, 11 (73%) raced 306+/-67 days after injury and had greater mean (+/-SD) post-injury earnings/start ($5290.00+/-$8124.00) than pre-injury ($4971.00+/-$2842.00). Screw removal was not required in any horse. CONCLUSION: The AE screw is a viable option for repair of nondisplaced lateral condylar MC/MT3 fractures in Thoroughbred racehorses. CLINICAL RELEVANCE: Adequate stability of nondisplaced lateral condylar fractures can be achieved with a headless tapered compression screw while avoiding impingement on the collateral ligaments and joint capsule of the fetlock joint.     

Relationships between race earnings and horse age,sex, gait,track surface and number of race starts for Thoroughbred and Standardbred racehorses in North America

Reasons for performing study: There is no consensus on objective outcome measures that can be used to determine if a medical or surgical treatment affects race performance. Objective: To determine the association between 2 commonly used outcome measures (total starts and total earnings) and age, sex, gait and race surface. Methods: A cross‐sectional study was performed using the race performance data for all Thoroughbred horses age 2, 3, 4 and 5 years racing in the United States, and Standardbred horses of the same ages racing in the United States and Canada during the year 2006. Median earnings and starts were determined for each combination of age, sex and track surface (for Thoroughbred) or gait (for Standardbred). The effect these variables had on starts on race earnings ($) was determined using linear regression. Results: Race records for 68,649 Thoroughbreds and 25,830 Standardbreds were obtained. All independent variables (age, breed, sex, gait, track surface and total number of starts) had a significant impact on total earnings (P<0.0001). Conclusions: The data show considerable variation across age groups and track surfaces for Thoroughbreds and across age groups for Standardbreds. They also show that the decision to use earnings or starts as outcome measures could have a marked effect on reported success for a particular treatment. Potential relevance: Both earning and start data should be reported in studies evaluating outcome following surgery or other intervention. Considerations of age, breed, sex, track surface and gait should be included in the design of these studies.     

Racing and sales performance after unilateral or bilateral single transphyseal screw insertion for varus angular limb deformities of the carpus in 53 thoroughbreds

Objective: To evaluate the racing and sales performance of Thoroughbred horses with varus angular limb deformities of the carpus treated by unilateral or bilateral single transphyseal screw (STS) placement. Study Design: Case series. Animals: Thoroughbred horses (n=53). Methods: Medical records (January 1, 2005–December 31, 2006) of yearling Thoroughbreds treated for carpal angular limb deformity by transphyseal screw insertion in the distal aspect of the radius were reviewed. Retrieved data were sex, surgery, and screw removal dates, surgical site, appearance, limb(s) affected, type of angular limb deformity, and degree of angular deviation measured by a goniometer. Racing and sales data were collected for analysis from an online racing site for all treated horses and their maternal siblings. Results: No significant differences were identified between treated horses and their maternal siblings in yearling sale price, 2‐year‐olds in training sale price, percent starters, percent winners, and starts, earnings, and earnings/start made during the 2‐ and 3‐year old years. Conclusions: No deleterious effects on sales or racing performance were identified after use of STS in the distal aspect of the radius of Thoroughbreds for the treatment of varus angular limb deformities of the carpus.     

Heritability and complex segregation analysis of hypoadrenocorticism in the standard poodle

The heritability of hypoadrenocorticism (Addison's disease) was evaluated in 778 standard poodles with known Addisonian phenotypes. Addisonian status was confirmed clinically by adrenocorticotropic hormone (ACTH) challenge and 8.6 per cent of the poodles enrolled in the study were classified as being Addisonian. Hypoadrenocorticism affected both sexes with equal probability (P > 0.1). The most common coat colours had a negligible effect on the incidence of hypoadrenocorticism (P > 0.09), although red coat colour had a significant impact on the disease, probably due to the relatively small numbers of dogs with that coat colour. The heritability of hypoadrenocorticism in the standard poodle was estimated to be 0.75. Complex segregation analyses suggested that hypoadrenocorticism in the breed is influenced by an autosomal recessive locus. Clarification of both the heritability and mode of inheritance of hypoadrenocorticism in the standard poodle allows for better-informed breeding decisions.