Equine half sibs with an unbalanced X;15 translocation or trisomy 28.

This research paper discusses two unique chromosome abnormalities identified in sibling horses. The first horse, which was short and infertile, had a rare ‘unbalanced X;15 translocation’ problem, while the second, also unusually short, exhibited ‘trisomy 28,’ which is a genetic disorder.

Unbalanced X;15 Translocation in Case 1

• The analysis first focuses on a horse (referred to as Case 1) that was unusually short and infertile. The cause was traced back to a genetic abnormality where a section of the X chromosome had been relocated to chromosome 15. This type of genetic repositioning, known as “translocation”, is categorized as “unbalanced”, because there was significant loss from the X chromosome.
• Upon further investigation, this translocated X chromosome was found to replicate, or produce copies, later than expected. Late replication is often associated with complications and disorders.
• Interestingly, this late replication was found to seep variably into the autosomal (relating to any chromosome that isn’t a sex chromosome) segment attached to the relocated portion of the X chromosome.

Trisomy 28 in Case 2

• The second horse (referred to as Case 2) had another distinct abnormality, namely trisomy 28. In this condition, an extra copy of chromosome 28 was present in the horse’s genetic makeup, making it ‘trisomic’.
• Cryptorchidism, a condition where one or both of the testes fail to descend, was also found in this horse.

Additional Observations

• Further genetic examination of the horses’ parents and two other sibling horses showed normal genetic complements without any abnormality.
• The researchers also noted the birth of five healthy offspring with no chromosome irregularities, indicating that the previously identified abnormalities were likely random or ‘fortuitous’, rather than hereditary in nature.
• These two cases are particularly noteworthy as they represent the first-ever reported instance of unbalanced X-autosome translocation and autosome trisomy within the equine species.