Ullrich-Turner syndrome occurred in one of a pair of female twins. The chromosome constitution of the affected twin was 45,X/46,XX and that of the normal twin 46,XX. Investigation of banded chromosomes, red cell antigens, HLA types, red cell enzymes, and serum proteins indicates monozygosity. The twins are discordant for height, pterygium colli, ovarian function, strabismus, dental eruption, external ear formation, hearing loss, and performance scores on the Wechsler Intelligence Test. All of these differences can be attributed to X monosomy in one cell line in the affected twin, presumably resulting from mitotic nondisjunction or anaphase lag early during embryonic development. Ten other pairs of apparently monozygotic twins discordant for the Ullrich-Turner syndrome have been reported previously, and the findings in these cases are reviewed.

In five heterosexual male bovine twins and three freemartins, all not older than three months, XX/XY chimerism in the blood and gonads has been found. One heterosexual male bovine triplet showed at the age of 18 or 22 months XX/XY chimerism in the blood but not in the testes. In another male co-twin of a freemartin, XX/XY chimerism has been detected in the blood and the testes (at biopsy) at the age of two months. In the same animal, the blood chimerism was still present at the age of 18 months, whereas the chimerism of the testes had vanished at that time. Therefore, it can be concluded that, in the course of time, the XX cells of the testes have been eliminated. This elimination, which possibly occurs in embryonic life, partially explains why male co-twins of freemartins do not have malformations of the genital organs and are fertile. Possibly, the XY cells of the testes have an eliminating influence against neighbouring cells with an XX karyotype. The eliminating effect could be directed at the two active X chromosomes of these germ cells. The hypothesis would also account for the observation that blood lymphocytes with an XX karyotype, which have, according to the Lyon hypothesis, only one active X chromosome, are not eliminated with time.

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