Abstract
Background
A lumbosacral transitional vertebra (LTV) is a congenital anomaly with reported prevalences ranging from 0 to 67% in different dog breeds, implying possible genetic differences. LTV has been associated with canine hip dysplasia (CHD) and degenerative lumbosacral stenosis (DLSS). Genetic parameters, including heritability estimates, are important for understanding the genetic influence on specific traits and for evaluating the effectiveness of possible genetic selection in reducing the prevalence of disorders. This study aimed to determine the heritability of LTV in nine dog breeds in Norway and estimate the genetic correlation with CHD.
Results
The heritability estimates for LTV across the nine breeds ranged from low to moderate (0.056–0.314), while the heritability estimates for CHD were moderate to high (0.254–0.580). The estimates of genetic correlations between the two traits were mostly non-significant and varied strongly among breeds in size and direction.
Conclusions
This study indicated that genetic factors influence LTV in several breeds and that there is a potential to reduce the prevalence by genetic selection, even if the heritability estimates of LTV ranged from low to moderate. The heritability estimates of CHD were within the range reported earlier, ranging from moderate to high. There was no general indication of a genetic correlation between LTV and CHD across breeds.
Supplementary Information
The online version contains supplementary material available at 10.1186/s13028-025-00810-z.
Keywords: Canine, CHD, Heritability, Inheritance, LTV, Transitional vertebra
Background
A lumbosacral transitional vertebra (LTV) in dogs is a congenitally malformed vertebra located at the junction between the last normal lumbar vertebra and the sacrum [1]. The prevalence of LTV has been reported to vary by geographical location, classification system, and breeds included, ranging from 0 to 67% [1–8]. The significant difference in prevalences among dog breeds implies that genetic differences exist [2, 3, 7].
The heritability expresses how much of a trait’s phenotypic variance can be explained by additive genetic variance and ranges from 0 to 1. Heritability less than 0.2 is considered low; those with values between 0.2 and 0.5 are moderate, and those with values between 0.5 and 1.0 are highly heritable [9]. Generally, the higher the heritability, the stronger the influence of genetic effects on the trait and, thus, the greater the potential response to selective breeding [10].
The estimated heritability of canine hip dysplasia (CHD) has been reported to be > 0.20 in many breeds [11–15]. Systematic selection programs have resulted in significant genetic progress for many breeds and working dog populations [16, 17]. The heritability of LTV in German Shepherd dogs (GSD) has also been reported to be moderate to high [8, 18, 19], indicating a potential to reduce the prevalence through breeding [19]. However, limited information exists on the heritability of LTV in other breeds of dogs.
A recent study found that GSD with LTV type 1 have a higher risk of producing progeny with LTV types 2 and 3 compared to dogs with normal lumbosacral anatomy [18]. In that study, LTV was classified accordingly: LTV type 1 is characterised by an independent spinous process of the first sacral vertebra, which is separated from the medial sacral crest; LTV type 2 is a symmetrical form of a transitional vertebra, separated from the sacrum with an intervertebral space, and LTV type 3 is characterised by its asymmetrical morphology. LTV type 0 radiographically designates a dog with normal lumbosacral anatomy [20]. An eighth lumbar vertebra has been identified as a marker for LTV [6], with studies showing a high co-occurrence of this trait between parents and offspring. Research on the genetic basis of lumbosacral phenotypes suggests that both the presence of this vertebra and the fusion between the sacrum and first coccygeal vertebra should be included in LTV screening, as they likely share a common genetic origin [21].
Certain LTV types have been associated with CHD, asymmetric hip grades, and degenerative lumbosacral stenosis (DLSS) [2, 22–26]. Moreover, DLSS is a recognised cause for early retirement among service dogs in law enforcement and military roles [27, 28]. Additionally, CHD and DLSS are common causes of veterinary visits, where both diseases can lead to chronic pain, reduced quality of life, and altered behaviour [27–30]. Therefore, LTV may contribute to immediate and long-term discomfort and health issues, compromising overall animal welfare, with added emotional and financial burdens on owners [31, 32].
This study aimed to estimate the heritability (h²) for LTV and the additive genetic correlation between LTV and CHD in nine dog breeds in Norway. Identifying a positive correlation would suggest that breeding programs for CHD might also have a beneficial effect on the prevalences of LTV.
Methods
Data source
Breed selection was based on the evaluation of ventrodorsal (VD) radiographs from the Norwegian Kennel Club (NKK) database [33]. The database includes radiographs collected as part of the screening programme for CHD in Norway. Data from February 2014 to January 2022 were used, as this period ensured that radiographs were available in digital format, facilitating detailed and consistent classification of LTV [2] and enabling heritability estimation for LTV. More extensive historical data was available to estimate the heritability of CHD in the same dog breeds. Most breeds had data dating back to 1987: Brittany, English setter, Gordon setter, Norwegian Elkhound black and grey, and GSD; 1997: Eurasier and Portuguese Water dog; 2000: Danish - Swedish Farm dog.
Breed selection
Based on a previous study of 14 dog breeds evaluated for the prevalence of LTV [2], nine breeds with more than 600 dogs were evaluated for LTV-status. The selection included a high LTV prevalence (> 20%): Brittany, Eurasier, Norwegian Elkhound black, and Portuguese Water dog. Additionally, the Norwegian Elkhound grey, closely related to the black variant, was included for comparison. Two setter breeds, the English setter and the Gordon setter, were selected for inter-breed comparisons. GSD was included in a comparison with previous studies on LTV heritability. Lastly, the Danish-Swedish Farm dog, a breed weighing < 10 kg, was included to represent a small-sized dogs.
Inclusion criteria
Dogs were included in the study if they met the following criteria: hip radiographs and CHD grading according to the Fédération Cynologique Internationale (FCI) system [34]. Complete metadata, including a unique identity number, birth date, sex, and radiology date, were required [2]. According to CHD screening guidelines, dogs had to be aged ≥ 12 months at the time of radiography [2, 34].
Classification of lumbosacral transitional vertebra (LTV)
LTV was classified into four types: LTV 0 represents normal lumbosacral anatomy; LTV 1 is characterised by an independent spinous process of the first sacral vertebra, separated from the medial sacral crest; LTV 2 is a symmetrical transitional vertebra separated from the sacrum by an intervertebral space; and LTV 3 exhibits asymmetrical morphology [20]. Radiographs were excluded if dense rectal faecal material, the os penis, or poor positioning obstructed relevant anatomy or if imaging quality was suboptimal due to artefacts, inadequate exposure, or distortion that hindered accurate classification. Additionally, radiographs were excluded if they lacked a visible last normal lumbar vertebra, which is essential as a reference for LTV classification. Two authors (JAB and CT) independently evaluated the radiographs and resolved any discrepancies through consensus. This information has been previously published [2].
Classification of canine hip dysplasia (CHD)
The hip joint status of each dog was evaluated using the FCI five-grade system. This system classifies hips from A (normal joint) to E (severe CHD) based on the Norberg angle, degree of subluxation, acetabulum shape and depth, and secondary signs of osteoarthritis [34]. Only dogs with proper sedation, identification, left and right markings, and correct positioning were included [34]. This data was harvested from the NKK database [33].
Estimation of pedigree-based heritability
LTV was recorded as a binary trait (unaffected/affected), which is justified because it seems that all phenotypes of LTV share a common genetic background [18]. CHD was recorded (A-E) as an ordered categorical trait (1–5) for the statistical analyses. Tables 1 and 2 show the number of records for LTV and CHD.
Table 1.
The number of included dogs screened for canine hip dysplasia (CHD) by breed
| Breed | CHD1 | C% | D% | E% |
|---|---|---|---|---|
| Brittany | 3813 | 27.7 | 9.8 | 0.6 |
| Danish - Swedish Farm dog | 2600 | 26.8 | 5.9 | 1.8 |
| English setter | 15,195 | 12.8 | 4.3 | 0.7 |
| Eurasier | 2241 | 11.8 | 4.7 | 1.9 |
| German Shepherd dog | 28,478 | 25.4 | 6.7 | 0.9 |
| Gordon Setter | 10,379 | 14.4 | 6.3 | 1.2 |
| Norwegian Elkhound black | 2572 | 11.0 | 4.1 | 0.7 |
| Norwegian Elkhound grey | 13,360 | 21.3 | 5.4 | 0.2 |
| Portuguese Water dog | 1875 | 19.1 | 4.8 | 0.8 |
| Total number | 80,513 |
The table provides information on the number of dogs within each breed with a canine hip dysplasia (CHD) grading obtained from the Norwegian Kennel Club (NKK) database [33]. Most breeds had data dating back to 1987: Brittany, English Setter, Gordon Setter, Norwegian Elkhound black and grey, and GSD; 1997: Eurasier and Portuguese Water dog; 2000: Danish-Swedish Farm dog. CHD was graded according to the Fédération Cynologique Internationale (FCI) system, where A and B are considered free (not included), C indicates mild, D indicates moderate, and E indicates severe CHD [34]. The data are provided as percentages of dogs diagnosed with CHD grades C, D, and E
1 The total number of dogs within each dog breed included
Table 2.
The number of dogs evaluated for lumbosacral transitional vertebra (LTV) by breed
| Breed | LTV* | LTV** | LTV*** (%) |
|---|---|---|---|
| Brittany | 1070 | 835 | 386 |
| (46.2) | |||
| Danish - Swedish Farm dog | 1398 | 1027 | 149 |
| (14.5) | |||
| English setter | 3521 | 2007 | 179 |
| (8.9) | |||
| Eurasier | 1084 | 794 | 167 |
| (21.0) | |||
| German Shepherd dog | 2843 | 1663 | 243 |
| (14.6) | |||
| Gordon setter | 1371 | 1209 | 117 |
| (9.7) | |||
| Norwegian Elkhound black | 692 | 679 | 195 |
| (28.6) | |||
| Norwegian Elkhound grey | 1916 | 1361 | 242 |
| (17.8) | |||
| Portuguese Water dog | 873 | 664 | 146 |
| (22.0) | |||
| Total number | 14 768 | 10 239 | 1822 |
| (17.8) |
The table provides information on the number of dogs within each breed evaluated for lumbosacral transitional vertebra (LTV) based on ventrodorsal radiographs. It includes the total number of dogs and the number of dogs with LTV types 1–3. Radiographic records from February 2014 to January 2022 were used. This period was chosen because radiographs were most likely available as digital pictures for a thorough classification of LTV [2]. LTV was classified as follows: LTV 0 represents normal lumbosacral anatomy; LTV 1 is characterised by an independent spinous process of the first sacral vertebra, separated from the medial sacral crest; LTV 2 is a symmetrical transitional vertebra separated from the sacrum by an intervertebral space; and LTV 3 exhibits asymmetrical morphology [20]
* The total number of dogs evaluated within each breed, before exclusion; ** The number of dogs within each breed included for evaluation of LTV; *** The number of dogs within each breed affected with LTV types 1–3; %, percentage
The supplementary materials (Supplementary 1) provide complete pedigree data and details on model development. The tables present breed-specific statistics on pedigree information for dogs registered with LTV and CHD.
For each of the nine breeds, pedigree-based variance components were estimated in a bi-variate model using the following linear model:
y = Ryear + Sex + Ryear*Sex + litter + a + e.
Where: y = CHD or LTV, Ryear = Fixed effect of year of CHD recording, Sex = Fixed effect of sex, Ryear*Sex = Fixed interaction between Ryear and Sex, litter = Random effect of litter, a = Random additive genetic effect, e = Random residual error.
The assumed variance structure for the bi-variate model is shown in the supplementary material (Supplementary 2) [35].
Heritabilities were computed as: 
, for i = CHD or LTV. The genetic correlation (rg) between LTV and CHD was computed as: 
. The asymptotic standard error of the estimated heritabilities and genetic correlations were computed using the AI matrix and a Taylor series approximation.
Results
The heritability of LTV was low to moderate, ranging from 0.056 in the GSD to 0.314 in the Brittany, and was generally lower than for CHD (Table 3). Notably, the Norwegian Elkhound black (0.199 ± 0.087), Danish-Swedish Farm dog (0.124 ± 0.067), and English setter (0.121 ± 0.051) showed reasonable heritability, suggesting a stronger genetic component in these breeds. In contrast, the GSD had the lowest heritability (0.056 ± 0.051), indicating a weaker genetic contribution to LTV. The additive genetic variance for LTV was also low, ranging from 0.007 (GSD, Gordon setter) to 0.080 (Brittany). The Norwegian Elkhound black (0.042) and Eurasier (0.022) had relatively higher variance.
Table 3.
Heritability and genetic variances for hip dysplasia, lumbosacral transitional vertebra and their correlation, with covariances
| Breed | h2 CHD (SE) | Ad. gen. variance CHD1 (SE) | h2 LTV (SE) | Ad. gen. variance LTV2 (SE) | Gen. cor.3 (SE) | Cov.4 (SE) |
|---|---|---|---|---|---|---|
| Brittany | 0.37 | 0.406 | 0.31 | 0.080 | 0.13 | 0.02 |
| (0.04) | (0.06) | (0.10) | (0.03) | (0.17) | (0.03) | |
| Danish - Swedish Farm dog | 0.52 | 0.551 | 0.12 | 0.016 | -0.13 | -0.01 |
| (0.48) | (0.07) | (0.07) | (0.01) | (0.20) | (0.02) | |
| English setter | 0.32 | 0.253 | 0.12 | 0.010 | 0.21 | 0.01 |
| (0.02) | (0.02) | (0.05) | (0.00) | (0.18) | (0.01) | |
| Eurasier | 0.29 | 0.267 | 0.14 | 0.022 | -0.20 | -0.02 |
| (0.06) | (0.06) | (0.07) | (0.01) | (0.26) | (0.02) | |
| German Shepherd dog | 0.25 | 0.367 | 0.06 | 0.007 | 0.06 | 0.00 |
| (0.0) | (0.02) | (0.05) | (0.01) | (0.05) | (0.02) | |
| Gordon setter | 0.36 | 0.355 | 0.08 | 0.007 | 0.38 | 0.02 |
| (0.03) | (0.03) | (0.06) | (0.01) | (0.26) | (0.01) | |
| Norwegian Elkhound black | 0.42 | 0.301 | 0.20 | 0.042 | 0.62 | 0.07 |
| (0.05) | (0.05) | (0.09) | (0.02) | (0.19) | (0.02) | |
| Norwegian Elkhound grey | 0.37 | 0.261 | 0.09 | 0.012 | -0.05 | 0.00 |
| (0.02) | (0.02) | (0.05) | (0.01) | (0.20) | (0.01) | |
| Portuguese Water dog | 0.58 | 0.587 | 0.10 | 0.017 | -0.05 | -0.01 |
| (0.06) | (0.10) | (0.08) | (0.01) | (0.30) | (0.03) |
The table provides information regarding the heritability (h²) of canine hip dysplasia (CHD) and lumbosacral transitional vertebra (LTV) for each dog breed, along with their corresponding additive genetic variances. It also includes the genetic correlation between LTV and CHD and the corresponding covariance
1 Additive genetic variance CHD; 2 Additive genetic variance LTV; 3 Genetic correlation LTV-CHD; 4 Additive genetic covariance
SE, standard error; Gen, genetic
Heritability for CHD varied from 0.254 (GSD) to 0.580 (Portuguese Water dog) (Table 3). The Portuguese Water dog (0.580 ± 0.058) and Danish-Swedish Farm dog (0.515 ± 0.048) had the highest values, reinforcing a strong genetic influence. The GSD (0.254 ± 0.015) and Eurasier (0.288 ± 0.057) had the lowest heritability, suggesting a greater environmental impact. The additive genetic variance for CHD followed a similar trend, highest in the Portuguese Water dog (0.587) and Danish-Swedish Farm dog (0.551) and lowest in the English setter (0.253) and Norwegian Elkhound grey (0.261).
Genetic correlations between LTV and CHD varied across breeds, with some showing positive associations (e.g., Norwegian Elkhound black: 0.615 ± 0.193) and others negative or near-zero correlations (e.g., Eurasier: -0.199 ± 0.255, Danish-Swedish Farm dog: -0.132 ± 0.195) (Table 3).
Discussion
The first aim of this study was to estimate the heritability of LTV in nine dog breeds in Norway based on the same radiographs used for routine evaluation in the screening program for CHD. The heritabilities ranged from low to moderate, indicating that genetic improvements are possible by selective breeding strategies [9, 10]. The low heritability estimate for LTV in GSD is not consistent with previous studies where the heritability has been reported as moderate to high [8, 18, 19]. The prevalence of LTV in GSD in this study was also lower than previously reported [2, 4, 18]. The lower prevalence of LTV in Norwegian GSD is unexpected, given the presumed high exchange of breeding dogs between countries and the lack of selection for this trait in Norway. This variation may arise due to sampling differences, the classification system used, the statistical method employed for estimation, and the sample sizes [9, 10].
The second aim was to estimate the genetic correlation between LTV and CHD among the nine dog breeds. A positive genetic correlation would suggest that a breeding program targeting CHD could potentially reduce the prevalence of LTV. The results indicated breed differences, with both positive and negative correlations. This variability indicates that LTV and CHD do not share a consistent genetic basis across breeds, meaning one trait is not a reliable predictor of the other. Therefore, selection against one trait is unlikely to impact the other significantly, and both traits should be managed independently in breeding programs [36].
LTV type 2 and type 3 may have clinical implications related to CHD [2, 22, 23] and DLSS [24–26]. The clinical impact of CHD combined with lower back pain related to these LTV types remains unknown. However, these conditions - either independently or in combination - could have a negative impact on animal welfare and potentially lead to early retirement in working dogs [27–30].
A limitation of this study is that the pedigree-based LTV estimates for each breed are based on relatively small sample sizes. The low number of dogs in each of the original LTV classifications (Types 1–3) may reduce the statistical power of the model. To address this, we condensed these categories into a single “affected” group, increasing the number of observations per category and thereby enhancing statistical power. However, this binary classification, combined with reporting heritability on the observed rather than the underlying scale, may lead to a lower heritability estimate [19, 37]. Grouping LTV Types 1–3 as “affected” may be preferable, as they likely do not have a linear relationship and may share a common genetic background [18]. If LTV is to be included in systematic breeding programs, alternative models for estimating heritability should be explored to optimize and improve the precision of expected breeding progress estimates.
When the total population size is unknown, we cannot confirm that our data represents the whole population. However, we assume that most breeding dogs undergo hip dysplasia screening. Thus, our results are most likely valid for the breeding population, which forms the genetic basis for the next generation. We have no evidence to suggest that the prevalence of LTV differs between screened breeding dogs and unscreened dogs of the same breed.
This study relied exclusively on VD radiographs to classify LTV using a previously established classification system [20]. This system was chosen, as it is a straightforward system and its requirements are based on standard FCI radiographs for CHD screening, which was the only radiographic view available in the dataset.
Consequently, the terminology “lumbarisation” and “sacralisation” is not used [1]. It is important to note that the use of additional radiographic views or an alternative classification system for LTV could have yielded different results.
Conclusions
This study demonstrates variation in the heritability of LTV and the genetic correlation with CHD in nine dog breeds in Norway, indicating that there is no consistent correlation between the two traits. LTV heritability is low to moderate, but genetic improvement would be feasible if breeders choose to put weight on the trait in the selection of breeding dogs. Given the small population size of the Norwegian Elkhound black, a primarily local breed with a small population size, routine evaluation and selection against CHD must be balanced with measures to reduce LTV prevalence and maintain genetic diversity. Overall, with accurate LTV grading, the heritability estimates indicate that it would be possible to reduce the prevalence of LTV using genetic selection if kennel clubs and dog breeders would sign up for such a program.
Electronic supplementary material
Below is the link to the electronic supplementary material.
Acknowledgements
AniCura Jeløy Dyresykehus and The Research Council of Norway.
Author contributions
JAB contributed to the conceptualisation, formal analysis, data interpretation, original draft writing, and manuscript review and editing. BKS was involved in the conceptualisation, review and editing of the manuscript. CT contributed to conceptualisation, data interpretation, and manuscript review and editing. PM conducted the bi-variate genetic analysis. FL was responsible for conceptualisation, analysis preparation, data interpretation, and manuscript review and editing. All authors have read and approved the final version of the manuscript.
Funding
This study was financially supported by AniCura Jeløy Dyresykehus and The Research Council of Norway (project number 310847).
Data availability
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Declarations
Ethical approval
This study did not require official or institutional ethical approval.
Prior publication
Data have not been published previously.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
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Supplementary Materials
Data Availability Statement
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.