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Androgen Insensitivity Syndrome (AIS Horse)

Infertility and external female genitalia in male horses


  • Affected horses (genotypic males) have external female genitalia.
  • Underdeveloped uterus and underdeveloped, retained testes
  • Infertility
  • Typical male-like behavior

General Information

  • Affected foals appear like normal females.
  • Underdeveloped male and female genitalia result in infertility.
  • Retained testes are usually surgically removed.
  • A malfunction of androgen receptor (male steroid hormone) causes the syndrome.
  • No other effects on health of affected horses are known.

Breeds affected

Quarter Horse and related breeds

Genotype and Lab Report

Inheritance: X-chromosomal recessive

→ The affected gene is located on the X-chromosome. Females have two versions of this chromosome while males only have one. Males with this variant are affected by AIS. Females can be carriers of this variant (heterozygous, X/X-ais).




X/X = normal female

The mare has no genetic variant causative for the AIS and therefore cannot pass it on to its offspring.


X/X-ais = carrier female

The mare is clinically healthy. The genetic variant will be passed on to its offspring with a probability of 50%. If the variant is passed on to male offspring, the foal will be an affected male.


X/Y = normal male

The male has no genetic variant causative for the AIS.


X-ais/Y = affected male

The male appears to be a normal female but has the male genotype (XY) and is infertile.



  • Offspring of a carrier female should be tested for their genotype (X/X or X/X-ais or X-ais/Y → affected male).
  • Carrier females (X/X-ais) may be used for breeding but their male offspring have a 50% of being affected.
  • Affected males (X-ais/Y) are infertile but otherwise healthy.

Test information

This test detects the single basepair change in the AR Gen.


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 Online Mendelian Inheritance in Animals.

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