MONOZYGOTIC TWINS WITH DISCORDANT SEX

A nine-year-old girl with short stature was referred to the department of pediatrics at Kyushu University. The clinical diagnosis was Turner syndrome; karyotypic analysis performed on peripheral blood, using GT G techniques, demonstrated a 45,X/47,XYY (17:83) mosaicism. Her twin b rother, a phenotypically normal male, had the same karyotype; 45,X/47, XYY (3:97) on peripheral blood. Their skin fibroblast karyotypes showe d the same mosaicism, ie. 45,X/47,XYY (41:59 and 31:69 respectively). On eleven biochemical genetic markers the twin pair were concordant, t hus the likelihood of monozygosity was 0.99527034. In addition, the an alysis of variable number of tandem repeat (VNTR) markers revealed the likelihood of monozygosity to be 0.99944386. The most plausible expla nation of the X/XYY mosaicism was nondisjunction of the Y in the first cleavage division of the 46,XY zygote. A disproportionate rate of cel l populations with 45,X and 47,XYY in the twinning process of the X/XY Y embryo, especially in the germ lines, would result in discordant sex in twin pairs.