a human sex chromosomal aberration characterized by the presence of three X chromosomes and two Barr bodies instead of the normal XX complement, so that somatic cells contain a total of 47 chromosomes; trisomy X. The condition occurs approximately once in every 1000 live female births and is confirmed diagnostically by the presence of the extra Barr body in the cells. Individuals with the anomaly show no significant clinical manifestations, although there is usually some degree of mental retardation. Because selective migration of the X chromosome occurs during meiosis, half of the offspring of a trisomy X female will be both chromosomally and phenotypically normal. Also called triple X syndrome.