XY sex reversal (SRY-related)

Gene: SRY

Transmission: Y-chromosome linked

Mutation: Deletion, SRY gene: deletions of variable size that remove the SRY gene from the Y chromosome.

Medical system: Reproductive

Breeds: Thoroughbred, Standardbred, Quarter Horse, Arab

Age of onset of symptoms: Present at birth, often not recognized until after puberty.

Infertility in the mare can generally be caused by environmental factors which can be treated or by genetic factors which cannot be treated.  The availability of molecular techniques will be increasingly useful for diagnosing genetic causes in cases of unknown infertility in the mare.

Chromosomal anomalies in the horse are observed to be of sporadic but relatively common occurrence.  A viable subclass of chromosomal anomalies involves the sex chromosomes.  Of note is the 64, XY mare, where the presence of XY sex chromosomes is discordant with the female phenotypic sex of the animal.  This has been observed in a number of horse breeds.  XY sex reversed females can have a fully feminized external phenotype and are usually diagnosed only after infertility problems appear as adults.  Alternatively, the sex reversed females can have a masculinized external phenotype that is evident at a younger age.  XY females generally have a small uterus and ovaries and are infertile.  Almost all of the XY female horses studied at the molecular level showed a variably sized deletion of the SRY gene.  The SRY (sex-determining region, Y) gene is the mammalian sex determining genetic factor located on the Y chromosome.  It is expressed in the indifferent primitive gonads during early embryonic development, inducing them to develop as testes.  It should be noted that some XY sex reversed mares were found to be SRY positive, and the cause of their sex reversal remains unknown.

References:

OMIA link: [1230-9796]

Bugno-Poniewierska M, Jankowska M, Raudsepp T, et al. (2024) Molecular cytogenetic screening of sex chromosome abnormalities in young horse populations. Equine Vet J 56:786-795.  [pm/38785417]

Raudsepp T, Durkin K, Lear TL, et al. (2010) Molecular heterogeneity of XY sex reversal in horses. Anim Genet 41 Suppl 2:41-52. [pm/21070275]

Pailhoux E, Cribiu EP, Parma P, Cotinot C. (1995) Molecular analysis of an XY mare with gonadal dysgenesis. Hereditas 122:109-112. [pm/7558880]

With contributions by: Ana-Gloria Barcelo Jimenez and Maude Cadoret, Class of 2030, Faculty of Veterinary Medicine, University of Montreal.  (Translation: DWS).