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Context

• In agricultural settings, because freemartins can’t reproduce, farmers often identify them through physical and/or behavioural traits.

Freemartin

• A freemartin is an infertile female cattle with masculinized behavior and non-functioning ovaries.
• Phenotypically, the animal appears female, but various aspects of female reproductive development are altered due to acquisition of anti-Müllerian hormone from the male twin.
• Genetically, the animal is chimeric: karyotypy of a sample of cells shows XX/XY chromosomes.
• The animal originates as a female (XX), but acquires the male (XY) component in utero by exchange of some cellular material from a male twin, via vascular connections between placentas: an example of microchimerism.
• The chimerism is mainly present in the hematopoietic stem cells.

Mechanism

• In most cattle twins, the blood vessels in the chorions become interconnected, creating a shared circulation for both twins.
• If both foetuses are the same sex this is of no significance, but if they are different, male hormones pass from the male twin to the female twin.
• The male hormones (testosterone and anti-Müllerian hormone) then masculinize the female twin, and the result is a freemartin.
• The degree of masculinization is greater if the fusion occurs earlier in the pregnancy – in about ten percent of cases no fusion takes place and the female remains fertile.
• The male twin is largely unaffected by the fusion, although the size of the testicles may be slightly reduced.
• Testicle size is associated with fertility, so there may be some reduction in bull fertility.
• Freemartins behave and grow in a similar way to castrated male cattle (steers).

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