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== Aetiology ==
== Aetiology ==
* Results from sexual modification of a '''female twin''' by the in-utero exchange of blood from a male fooetus.
* Results from sexual modification of a '''female twin''' by the in-utero exchange of blood from a male fooetus.
* Freemartin is XX genetic female, but becomes an XX/XY chimera.
* Freemartin is XX genetic female, but becomes an XX/XY chimera.
* 90% of female calves born co-twin to a male will be freemartins.
* There are two theories to explain this:
** '''Hormonal Theory'''
***Hormones from the male twin reach the female through vascular anastamoses between the fused placentae to cause masculinisation of the female gonad.
** '''Cellular theory'''
*** Exhange of blood-forming cells and germ cells between foetuses.
*** Reciprocal exchange results in identical erythrocyte antigen types and sex chromosome chimerism (60 XX/XY).
* Paramesonephric (Mullerian) ducts do not form or atrophy.
* Mesonephric (Wolffian) ducts persist in the female twin and may form rudimentary male tubular genitalia.
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== Diagnosis ==
== Diagnosis ==
* Clinical genital abnormalities
* Presence of sex chromatin bodies in circulating leukocytes of the male co-twin
* Chimerism (XX/XY) of haemopoeitic cells. Blood tests can be used to demonstrate different chromosome complement in nucleated WBC and presence of two blood groups in each twin.
* Rectal palpation and vaginoscopy reveal a shortened vagina
** In calves 1-4 weeks old, the vagina is normally 13-15cm in length compared to 5-6cm in a freemartin.
* Vulva and vestibule patent but vestibule or vagina is blind-ended - can be checked using speculum or probe.
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== Morphology ==
== Morphology ==
* Gonads range from modified Ovaries to structures resembling Testes which are intra-abdominal and do not descend through the inguinal canal (Cryptorchid).
* Gonads range from modified Ovaries to structures resembling Testes which are intra-abdominal and do not descend through the inguinal canal ('''Cryptorchid''').
* No spermatogenesis, sterile.
* Produce Testosterone as the major hormone.
* Rudimentary Uterus and Small Vagina.
* Enlarged clitoris, long tufts of hair round vulva, skin fold from groin to umbilicus.
* Vagina never communicates with uterus
* fibrous cords or non-patent uterine tubes.
* Ovaries are usually small, cord-like thickenings in the ovarian ligament are semniferous tubules.