Dwarfism

Dwarfism is an inherited genetic disorder affecting the Friesian horse breed. The mutation is thought to have been facilitated by a small number of founder individuals, the overall limited population size, low genetic diversity, and inbreeding during the formation of the breed's gene pool.

Affected horses have impaired growth of the limbs and ribs, resulting in the development of a disproportionate form of dwarfism. Over time, hyperextension and dropping of the fetlock joint occur due to weakening or damage of the muscles and tendons that normally provide support to the joint. The degree of fetlock laxity can vary in severity and often causes significant difficulties with movement. Excessive laxity of the tendons responsible for flexion of the limbs becomes more pronounced with increasing age.

Due to abnormal physiological development of the limbs, affected horses develop an abnormal gait with an atypically increased outward rotation in the regions of the hock and carpal joints. The thorax also exhibits characteristic abnormalities, resulting in a broader chest and narrowing at the junction between the cartilaginous and bony portions of the ribs. The head is comparable in size to that of healthy individuals. The back is elongated and the legs are disproportionately short. These defects lead to poor development and weakness of the muscles.

Dwarfism is an autosomal recessive inherited disorder caused by a mutation in the B4GALT7 gene (g.3772591C>T), which leads to reduced gene expression and consequently has a negative effect on bone development. The disease develops in individuals that inherit the mutated allele from both parents. These individuals are designated P/P (positive/positive). Carriers of the mutated allele, designated N/P (negative/positive), have inherited the mutation from only one parent and show no clinical signs. However, they can pass the mutation on to their offspring.

When two heterozygous carriers (N/P) are mated, theoretically 25% of the offspring will be clear, 50% will be carriers, and 25% will inherit the mutated allele from both parents and be affected by dwarfism. Mating a clear individual (N/N) with a carrier (N/P) theoretically results in 50% carriers and 50% clear offspring. If a carrier (N/P) is mated with an affected individual (P/P), theoretically 50% of the offspring will be affected and 50% will be carriers.

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References:
Leegwater, P.A., Vos-Loohuis, M., Ducro, B.J. et al. “Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7.” BMC Genomics 17, 839 (2016).
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. and Brama, P.A.J. (2010). “Genome-wide SNP association–based localization of a dwarfism gene in Friesian dwarf horses.” Animal Genetics 41: 2–7.