We describe a prenatally detected case of double trisomy involving chromosome 21 and the X chromosome (48, XXX, + 21) along with determination of the segregation errors responsible for the double aneuploidy. The patient was ascertained as a result of an abnormal maternal serum analyte screen showing an increased risk for fetal Down's syndrome. Following determination of the abnormal karyotype, pregnancy termination was elected. Microsatellite polymorphisms and cytogenetic heteromorphisms were used to determine that both aneuploidies arose as a result of non-disjunction in maternal meiosis II. These results support hypotheses that a segregation defect at a cellular level may cause non-disjunction involving more than one chromosome.
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