Thirty-four heifers not conceiving to three or more inseminations or natural service were bled and karyotyped by leucocyte culture. Twelve were found to be sex-chromosome chimeras of which five were single-born. Three out of the five singleton sex-chromosome chimeric heifers were slaughtered and at post mortem examination abnormalities of the reproductive tract were observed. Two the the slaughtered animals were Friesian heifers that had shown normal oestrous cycles and had reproductive organs apparently normal on clinical examination; however, one of them with 5 per cent male cells had a nonpatent cervix, while the other with 12 per cent male cells had non-patent uterine horns. Both heifers possessed functional ovaries, the former had an active corpus luteum and the latter had a developing follicle. The third heifer with 45 per cent male cells had a normal vagina, enlarged clitoris, seminal vesicles and gonads resembling fetal testes with both primitive ovarian and testicular structures. The Old Gloucester and Friesian heifers with 2-5 per cent and 12-0 per cent male cells are alive and have normal external genitalia but rudimentary uterine horns with no palpable gonads. Cytogenetic examination has demonstrated for the first time the existence of single-born freemartins resulting from the death of their male co-twins after fetal vascular anastomosis has been established.
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