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A freemartin or free-martin (sometimes martin heifer) is an infertile female mammal with masculinized behavior and non-functioning ovaries. Genetically the animal is chimeric: Karyotyping 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. Externally, the animal appears female, but various aspects of female reproductive development are altered due to acquisition of anti-Müllerian hormone from the male twin. Freemartinism is the normal outcome of mixed-sex twins in all cattle species that have been studied, and it also occurs occasionally in other mammals including sheep, goats and pigs. == History == Freemartins are known to have been described by the Roman writer Varro, who called them "taura".〔(EARLY RECOGNITION OF THE FREEMARTIN CONDITION IN HEIFERS TWINBORN WITH BULLS ), by W.W. Swett, C.A. Matthews, and R. R. Graves, in the Journal of Agricultural Research, vol. 61, no. 8 (October 15, 1940)〕 The 18th-century physician John Hunter discovered that a freemartin always has a male twin. It was hypothesized early in the 20th century that masculinizing factors travel from the male twin to the female twin through the vascular connections of the placenta because of the vascular fusion and affect the internal anatomy of the female.〔(Nelson, Randy. An introduction to behavioral endocrinology. Sinauer Associates: Massachusetts. 2005: pg 145)〕 Several researchers made the discovery that a freemartin results when a female fetus has its chorion fuse in the uterus with that of a male twin. The result was published in 1916 by Tandler and Keller.〔Keller, K. and Tandler, J.: Wiener Tieraztl. Wochensch., 3, 513-526 (1916).〕 The discovery was made independently by American biologist Frank R. Lillie, who published it in Science in 1916. Both teams are now credited with the discovery. In rural areas folklore often claimed this condition was not just peculiar to cattle, but extended also to human twins; this belief perpetuated for generations, as was mentioned in the writings of Bede.〔de Albuquerque, Martim ''Notes and Queries'' Volume 2. 1857 by Oxford University Press, p. 149〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「freemartin」の詳細全文を読む スポンサード リンク
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