Reproduction: gonadal dysgenesis

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  • Gonadal dysgenesis results from chromosomal abnormalities in females.
  • Cause :
  • Genetic abnormality of the chromosome resulting in range of phenotypic abnormalities of the genitalia.
  • Most commonly reported chromosome abnormality is 63XO (missing sex chromosome).
  • Klinefelter's syndrome (rare) occurs when an extra X chromosome occurs with an XY; the individual is phenotypically male.
  • Intersex occurs in XX females with masculinizing gene either mutated or collected from the Y chromosome.
  • Signs : infantile reproductive organs and infertility.
  • Diagnosis : karyotyping; can be difficult to diagnose phenotypically before puberty.
  • Treatment : none available.
  • Prognosis : permanent infertility.


Differential diagnosis


Clinical signs

  • Intersex horses display a range of phenotypes from (almost) complete male to complete female.
  • The most common presentation of intersex is a mare with an enlarged clitoris and variously developed abdominal or inguinal testes.
  • XXY syndrome results in inhibited testicular development - not evident before puberty.
  • Aggressive or male behavior.
  • External genitalia normal although vulva may be small. No clitoral enlargement.
  • Small for breed (especially 63XO).
  • Phenotypic expression in XY sex reversal syndrome is highly variable - feminine to masculine mare.
  • Poor conformation    .
  • XXY mosaics and XY sex reversal may show masculine features.
  • Occasional XY sex reversal cases are fertile.
  • Other chromosomal abnormalities are phenotypically similar to X monosomy although occasionally they have functional ovaries, ovulate and even conceive.



  • Mares with confirmed gonadal dysgenesis, especially 63XO, are usually infertile but occasional cases have been reported as fertile.
  • Some mosaic types may be capable of limited breeding.

Reasons for treatment failure

  • Most mares with this condition are infertile.
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