Friesian Horse Dwarfism

Quick Summary

Dwarfism in Friesians is an inherited disorder characterized by a disproportionate growth with reduced bone length of limbs and ribs while the size of the head and length of the back are normal.

Phenotype: Dwarfism is characterized by a disproportionate growth with reduced bone length of limbs and ribs while the size of the head and length of the back are normal. Affected horses have hyperextension of fetlock joints of all limbs with varying degrees of severity.

Mode of Inheritance: Autosomal recessive

Alleles: N = Normal/Unaffected, D = Dwarfism

Breeds appropriate for testing: Friesian, Friesian crosses

Explanation of Results:

  • Horses with N/N genotype will not be affected by Friesian dwarfism and cannot transmit this dwarfism variant to their offspring.
  • Horses with D/N genotype will not be affected by Friesian dwarfism, but are carriers. They may transmit this dwarfism variant to 50% of their offspring. Matings between two carriers result in a 25% chance of producing a foal with Friesian dwarfism.
  • Horses with D/D genotype will have Friesian dwarfism.
Price

$40 one test per animal
$60 this test + one test from list below

$70 entire Friesian Health Panel (all 3 tests)

Panels Available
Additional Details

Dwarfism in Friesians is characterized by a disproportionate growth with reduced bone length of limbs and ribs while the size of the head and length of back are normal. Microscopic analysis of growth plates in affected animals shows irregular transition from cartilage to bone and abnormal arrangements of chondrocytes (cartilage cells). Affected horses have hyperextension of fetlock joints of all limbs with varying degrees of severity. A mutation (c.50G>A) in exon 1 of the Beta-1,4-Galactosyltransferase 7 (B4GALT7) gene has been identified that is associated with this disorder. B4GALT7 plays an important role in proper formation of extracellular matrix, a key element for bone development. The mutation affects normal functioning of the gene which interferes with proper collagen formation.

Friesian dwarfism is inherited as a simple autosomal recessive trait. This means that affected foals have two copies of the defective gene and that the disorder occurs in males and females. It is estimated that about 12% of the Friesian population are carriers (N/D) of the mutation.

Testing for Friesian dwarfism assists owners and breeders to identify carriers and to make informed mate selection to avoid producing affected foals. Matings between carriers have a 25% chance of producing affected foals. Breeding carriers to normal horses is safe and a means to maintain genetic diversity in the breed and avoid production of affected foals.

Turnaround Time
5-10 business days
Type of Sample

Species

Type of Test

Results Reported As
Test Result Dwarfism
N/N No copies of the dwarfism mutation. Horse is normal.
N/D

1 copy of the dwarfism mutation. Horse is normal but a carrier.

D/D

2 copies of a dwarfism mutation. Horse is affected.

References

Back, W., Lugt, J. J., Nikkels, P. G., Belt, A. J., Kolk, J. H., & Stout, T. A. (2008). Phenotypic diagnosis of dwarfism in six Friesian horses. Equine Veterinary Journal, 40(3), 282-287. doi: 10.2746/042516408x278201

Orr, N., Back, W., Gu, J., Leegwater, P., Govindarajan, P., Conroy, J., Ducro, B., Van Arendonk, J. A. M., MacHugh, D. E., Ennis, S., Hill, E. W., & Brama, P. A. (2010). Genome-wide SNP association-based localization of a dwarfism gene in Friesian dwarf horses. Animal Genetics, 41, 2-7. doi: 10.1111/j.1365-2052.2010.02091.x

Boerma, S., Back, W., & Oldruitenborgh-Oosterbaan, M. M. (2011). The Friesian horse breed: A clinical challenge to the equine veterinarian? Equine Veterinary Education, 24(2), 66-71. doi: 10.1111/j.2042-3292.2011.00302.x

Leegwater, P. A., Vos-Loohuis, M., Ducro, B. J., Boegheim, I. J., Steenbeek, F. G., Nijman, I. Monroe, G. R., Bastiaansen, J. W., Dibbits, B. W., Back, W., J., & Schurink, A. (2016). Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7. BMC Genomics, 17(1). doi: 10.1186/s12864-016-3186-0