Breeds Affected: Quarter Horse
Samples Accepted: Blood, Hair
Disease Information: Androgen Insensitivity Syndrome (AIS) in Quarter Horses occurs when a mutation affects the androgen receptor. The androgen receptor is unable to respond properly to the androgen hormone, so the animal does not become masculinized. This results in XY (chromosomally male) horses with female external genitalia. The affected Quarter Horses appear to be female, but are unable to reproduce because they have no functional female reproductive system, and their testes are internal.
Inheritance Information: AIS is X-linked recessive, because the androgen receptor gene (AR) is located on the X-chromosome.
The possible genotypes are:
XX This horse is a normal female.
XXAIS This horse is a carrier female. She is physically normal, but will pass the mutation (XAIS) to 50% of her offspring. When bred to a normal male, she can have foals of all four genotypes: normal female (XX), carrier female (XXAIS), normal male (XY), and affected male (XAISY).
XY This horse is a normal male.
XAISY This horse appears to be female, but is actually an affected male. It is infertile.
– Female offspring of a carrier female should be genetically tested to determine if they are females (XX, XXAIS) or affected males (XAISY).
– Carrier females (XXAIS) may be bred, but 50% of her male offspring will be infertile and have external female genitalia.
– Affected males are infertile and may show typical male aggressive behaviour around females. They are usually otherwise healthy.
Test Information: This mutation test identifies a single base change in the AR gene.
Révay, T., Villagómez, D.A., Brewer, D., Chenier, T., King, W.A.: GTG mutation in the start codon of the androgen receptor gene in a family of horses with 64,XY disorder of sex development. Sex Dev 6:108-16, 2012. Pubmed reference: 22095250. DOI: 10.1159/000334049.
Further information is available at the Online Mendelian Inheritance in Animals website.