Little Leg (Long Back) Fitness Structure & Genetics

Structure & Genetics

Dogs come in a remarkable range of body shapes and proportions. These differences are strongly influenced by genetics and generations of selective breeding.

As a canine fitness professional, my role is not to diagnose genetic conditions or provide medical advice. However, understanding some of the factors that influence body structure can help us make more informed decisions when implementing a canine fitness program.

In this lesson, we will review some structural and genetic traits associated with shortened limbs in dogs. The goal is not to become experts in genetics. It is to understand how these traits may relate to body structure and why exercise selection should reflect the individual dog.

Long Backs or Short Legs?

Dogs with short legs are often described as “long-backed.”. However, this appearance typically reflects shortened limbs rather than an unusually long vertebral column, which can make the back appear long relative to the dog’s height.

This description reflects the relationship between body length and height: when limb length is reduced, the trunk can appear longer relative to the dog’s overall height.

In dogs with disproportionate dwarf phenotypes, shortened limbs reflect altered skeletal development involving the long bones of the limbs. However, visual appearance alone cannot determine why a dog has particular proportions or which genetic variants the dog carries.

Describing Body Proportions

One simple way to describe a dog’s overall proportions is to compare body length and height.

Body Length
Measure from the point of the shoulder to the ischial tuberosity of the pelvis.

Body Height
Measure from the top of the withers to the ground.

If a dog’s body length is noticeably greater than its height, the dog may appear rectangular, “long-backed,” or short-legged in overall conformation.

It’s important to remember that many dogs naturally have rectangular body proportions. Being longer than tall does not by itself indicate a dwarf phenotype, and body proportions alone cannot confirm whether a dog carries variants associated with chondrodysplasia, chondrodystrophy, or another skeletal trait.

Dwarfism and FGF4-Related Traits

Canine dwarfism is not one condition or one genetic mutation. It can be broadly described as proportionate or disproportionate.

In proportionate dwarfism, a dog is smaller than expected, but the different regions of the body remain relatively proportional.

In disproportionate dwarfism, some parts of the skeleton are affected more than others. For example, the limbs may be shortened relative to the trunk. This is the type of dwarfism most relevant to the little-legged breeds addressed in this course.

FGF4 is a gene involved in early development, including development of the limbs. Researchers have identified several FGF4 retrogenes in canids. Retrogenes are additional gene copies created through a natural copying and insertion process.

Two functional FGF4 retrogenes are especially relevant to this lesson:

FGF4L1, located on chromosome 18
FGF4L2, located on chromosome 12

Both are associated with shortened limbs. The UC Davis Veterinary Genetics Laboratory labels the FGF4L1-associated variant and trait as CDPA and the FGF4L2-associated variant and trait as CDDY.

Chondrodysplasia: CDPA

In this lesson, chondrodysplasia, or CDPA, refers to the short-legged phenotype associated with FGF4L1 on chromosome 18. This retrogene is associated with reduced growth of several limb bones, resulting in disproportionately short legs.

In some chondrodysplastic breeds, premature closure of the distal ulnar growth plate has been described as a suspected hereditary contributor to angular forelimb deformity. If the ulna stops growing before the radius, it becomes shorter relative to the radius. As the radius continues to grow within the space limited by the shorter ulna, it may curve. This may be accompanied by radial torsion and/or carpal valgus.

Based on current evidence, CDPA is not considered an independent cause of the premature intervertebral disc degeneration associated with CDDY.

Chondrodystrophy: CDDY

In this lesson, chondrodystrophy, or CDDY, refers to the phenotype associated with FGF4L2 on chromosome 12. This phenotype includes shortened limbs and premature degeneration of the intervertebral discs.

Premature degeneration can make the intervertebral discs more susceptible to Hansen type I disc extrusion. However, degeneration and extrusion are not the same.

A dog can have CDDY-associated disc degeneration without a recognized herniation, apparent pain, or neurological impairment. A CDDY result indicates increased risk, but it cannot predict whether an individual dog will experience disc herniation, when it might occur, or how severe any resulting clinical signs might be.

The term intervertebral disc disease, or IVDD, can be confusing because it is often used as an umbrella term for several related but distinct concepts. For clarity, this lesson uses the following definitions:

Disc degeneration refers to changes in the structure and composition of an intervertebral disc.
Disc herniation refers to disc material moving beyond its normal location. Extrusion and protrusion are different forms of disc herniation.
Clinical signs are the effects a dog may experience, such as spinal pain, weakness, altered coordination, or other neurological impairment.

Disc degeneration, disc herniation, and clinical signs may be related, but they are not interchangeable. Degeneration does not necessarily mean that herniation or clinical signs are present.

Genetic testing note: Genetic testing can determine whether a dog has FGF4L1, FGF4L2, or both, as well as the number of copies detected. However, a negative CDPA or CDDY result does not rule out other genetic causes of shortened limbs or other skeletal, orthopedic, or neurological conditions.

Why This Matters for “Dwarf” Dog Breeds

Both CDPA & CDDY retrogenes have been identified in populations of Dachshunds, Basset Hounds, and Pembroke and Cardigan Welsh Corgis, among other breeds. However, breed and visible proportions alone cannot confirm an individual dog’s genotype.

Dogs within these breeds may share recognizable body proportions, but structure, posture, and movement can vary considerably among individuals. Body structure and spinal health are influenced by multiple genetic and non-genetic factors. Breed, appearance, and an FGF4 test result therefore provide only part of the picture.

The workouts in this course are designed for little-legged dogs and account for common structural and postural considerations, including angular limb deformity and carpal valgus. The exercises are progressed gradually and are intended to support trunk strength and spinal stability, distal limb strength, coordinated limb use, balance, body awareness, and whole-body strength.

These workouts are not intended to change skeletal conformation or to treat intervertebral disc degeneration or herniation. Exercise selection and progression should still be tailored to each dog’s structure, posture, movement, comfort, and function.

Scope Note

This lesson provides general education about body structure and genetics in dogs. It is not intended to diagnose a genetic condition, intervertebral disc disease, pain, or any orthopedic or neurological condition.

Genetics may influence structure, and structure may influence movement demands, but movement quality is also shaped by many other factors, including strength, coordination, training history, environment, pain status, and overall health.

If a dog shows pain, lameness, changes in gait or function, reluctance to move, weakness, altered coordination, paw scuffing, knuckling, or other possible neurological signs, pause the fitness program and consult a veterinarian.

Dogs with back or neck pain, a diagnosed or suspected disc herniation, neurological impairment, previous spinal surgery, or other current orthopedic or neurological concerns should follow exercise recommendations provided by their veterinarian or veterinary rehabilitation professional.

Terminology Note

Terminology for these traits has evolved and varies among sources. Bannasch et al. (2022) identify the FGF4 retrogenes on chromosome 18 and chromosome 12 as FGF4L1 and FGF4L2, respectively. The UC Davis Veterinary Genetics Laboratory uses CDPA for the FGF4L1-associated variant and trait and CDDY for the FGF4L2-associated variant and trait.

This lesson uses CDPA and CDDY in this specific genetic context. Other sources may identify the retrogenes by chromosome location or use the broader terms chondrodysplasia and chondrodystrophy somewhat differently.

Sources and Optional Further Reading

The selected resources below informed the scientific background of this lesson. They are provided for learners who would like to explore canine genetics, skeletal morphology, angular limb deformity, and intervertebral disc degeneration in greater depth.

Bannasch, D., Batcher, K., Leuthard, F., Bannasch, M., Hug, P., Marcellin-Little, D. J., Dickinson, P. J., Drögemüller, M., Drögemüller, C., & Leeb, T. (2022). The effects of FGF4 retrogenes on canine morphology. Genes, 13(2), Article 325. https://doi.org/10.3390/genes13020325
Batcher, K., Dickinson, P., Giuffrida, M., Sturges, B., Vernau, K., Knipe, M., Hadji Rasouliha, S., Drögemüller, C., Leeb, T., Maciejczyk, K., Jenkins, C. A., Mellersh, C., & Bannasch, D. (2019). Phenotypic effects of FGF4 retrogenes on intervertebral disc disease in dogs. Genes, 10(6), Article 435. https://doi.org/10.3390/genes10060435
Brown, E. A., Dickinson, P. J., Mansour, T., Sturges, B. K., Aguilar, M., Young, A. E., Korff, C., Lind, J., Ettinger, C. L., Varon, S., Pollard, R., Brown, C. T., Raudsepp, T., & Bannasch, D. L. (2017). FGF4 retrogene on CFA12 is responsible for chondrodystrophy and intervertebral disc disease in dogs. Proceedings of the National Academy of Sciences of the United States of America, 114(43), 11476–11481. https://doi.org/10.1073/pnas.1709082114
Dickinson, P. J., & Bannasch, D. L. (2020). Current understanding of the genetics of intervertebral disc degeneration. Frontiers in Veterinary Science, 7, Article 431. https://doi.org/10.3389/fvets.2020.00431
Lappalainen, A. K., Pulkkinen, H. S. M., Mölsä, S., Junnila, J., Hyytiäinen, H. K., & Laitinen-Vapaavuori, O. (2023). Breed-typical front limb angular deformity is associated with clinical findings in three chondrodysplastic dog breeds. Frontiers in Veterinary Science, 9, Article 1099903. https://doi.org/10.3389/fvets.2022.1099903
Olby, N. J., Moore, S. A., Brisson, B. A., Fenn, J., Flegel, T., Kortz, G. D., Lewis, M. J., & Tipold, A. (2022). ACVIM consensus statement on diagnosis and management of acute canine thoracolumbar intervertebral disc extrusion. Journal of Veterinary Internal Medicine, 36(5), 1570–1596. https://doi.org/10.1111/jvim.16480
Parker, H. G., VonHoldt, B. M., Quignon, P., Margulies, E. H., Shao, S., Mosher, D. S., Spady, T. C., Elkahloun, A., Cargill, M., Jones, P. G., Maslen, C. L., Acland, G. M., Sutter, N. B., Kuroki, K., Bustamante, C. D., Wayne, R. K., & Ostrander, E. A. (2009). An expressed FGF4 retrogene is associated with breed-defining chondrodysplasia in domestic dogs. Science, 325(5943), 995–998. https://doi.org/10.1126/science.1173275
Reunanen, V. L. J., Jokinen, T. S., Hytönen, M. K., Junnila, J. J. T., & Lappalainen, A. K. (2023). Evaluation of intervertebral disc degeneration in young adult asymptomatic Dachshunds with magnetic resonance imaging and radiography. Acta Veterinaria Scandinavica, 65, Article 42. https://doi.org/10.1186/s13028-023-00702-0
Veterinary Genetics Laboratory. (n.d.). Chondrodystrophy (CDDY and IVDD) and chondrodysplasia (CDPA). University of California, Davis. Retrieved July 31, 2026, from https://vgl.ucdavis.edu/test/cddy-cdpa

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