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Freemartinism is the most common form of intersexuality in cattle. It concerns a vast majority (over 90%) of females originating from heterosexual twins. The male co-twin to a freemartin is usually fertile and normally developed; however, some cases demonstrating abnormalities of the reproductive organs were also described.

Masculinization of the internal reproductive tract of these animals results from the exposure of the female foetus to the blood of the male twin in the uterus. Abnormalities of the female twin are most likely caused by the anti-Mullerian hormone (AMH). In addition to the transfer of hormones, anastomoses of placental vessels allow for the interchange of haemopoetic stem cells between foetuses. In heterosexual twins it results in the formation of sex chromosome chimerism (60, XX/XY) in peripheral blood leucocytes. Twinning in cattle ranges from about 1% for beef breeds to about 4% for dairy breeds. Freemartinism occurs more frequently in cattle than in any other species. Since freemartinism is responsible for congenital female infertility, its early diagnosis is recommended to avoid economic losses caused by useless therapeutical treatment.


Komisarek, J., and Dorynek, Z. (2002). Genetic aspects of twinning in cattle. Journal of Applied Genetics 43, 55–68.

Nowacka, J., Switonski, M., Mackowski, M., Slota, E., Radko, A., Zabek, T., and Urbaniak, K. (2004). The ambiguity of freemartinism diagnosis in cattle revealed by cytogenetic and molecular techniques. Czech Journal of Animal Science 49, 239–243.