A developmental anomaly in which complete distal bones (ulna and fibula) reappear with angular deformity, luxation and lameness. It is associated with large deletions in the pseudoautosomal region of the SHOX locus (which also affect part of the CRLF2 gene). Recessive inheritance described in the Shetland Pony. The test is complementary to clinical and radiographic examination.

Shetland Pony. Frequencies: limited data.

- Reappearance of complete distal bones (ulna/fibula)\n- Angular deformity of the limbs\n- Luxation and lameness\n- Altered gait

Rafati et al. (2016) characterized two large deletions (estimated at 160-180 kb and 60-80 kb) in the SHOX locus in affected Shetland ponies, with a complete association between the deletions and disease status verified in eight additional affected horses (PMID 27207956). The reference assembly of the pseudoautosomal region is highly fragmented, which complicates the design of a direct assay.

- Do not breed affected animals or those with imaging-confirmed malformation\n- Avoid matings between carriers if genotype is available\n- Review pedigrees of lines with cases\n- Confirm the feasibility of the assay with the laboratory (the region has a complex assembly)

Differential diagnosis from other dysplasias and dwarfisms (ACAN, B4GALT7) and acquired angular anomalies. Complementary: radiography of the limbs. The design of the molecular assay must be confirmed due to the complexity of the pseudoautosomal region.

1. Rafati N et al. 2016, grandes deleciones en el locus SHOX asociadas a atavismo esquelético en ponis de Shetland (PMID 27207956)
2. OMIA:002013 Atavismo esquelético del caballo