Reference Work Entry

Encyclopedia of Molecular Mechanisms of Disease

pp 2257-2257

X Polysomies, in Females

  • Deborah BartholdiAffiliated withInstitute of Medical Genetics, University of Zurich
  • , Albert SchinzelAffiliated withInstitute of Medical Genetics, University of Zurich


Trisomy X; Triple X; Tetrasomy X; Pentasomy X; 47, XXX; 48, XXXX; 49, XXXXX

Definition and Characteristics

47,XXX is characterized by an extra X chromosome that is of maternal origin in the majority of cases [1]. Females with X trisomy cannot be identified at any age by specific physical or behavioral features and might not come to medical attention, therefore, in the majority of cases. Trisomy X results from non-disjunction in meiosis I or II and rarely from post-zygotic non-disjunction. The mechanism underlying tetrasomy X and pentasomy seems to be successive non-disjunction in maternal meiosis I and II [2].


It has been estimated that 47, XXX occurs in approximately 1:1,000–1,200 female newborns. Unlike in 45, X, prenatal loss of 47, XXX fetuses after mid-gestation is rare. Only about 40 individuals with tetrasomy X have been described so far. Pentasomy X is even more rare.

Molecular and Systemic Pathophysiology

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