Trisomy X

Trisomy X
Other names	47,XXX, triple X syndrome, triplo-X syndrome, XXX syndrome
Three individuals with trisomy X
Specialty	Medical genetics
Symptoms	Tall stature, skeletal anomalies, minor neurocognitive and behavioural difficulties
Usual onset	Conception
Duration	Lifelong
Causes	Nondisjunction
Diagnostic method	Karyotype
Frequency	approximately 1 in 1,000 (female)

Trisomy X, also known as triple X syndrome and characterized by the karyotype^{[note 1]} 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 females, but is rarely diagnosed; fewer than 10% of those with the condition know they have it.

Those who have symptoms can have learning disabilities, mild dysmorphic features such as hypertelorism (wide-spaced eyes) and clinodactyly (incurved little fingers), early menopause, and increased height. As the symptoms of trisomy X are often not serious enough to prompt a karyotype test, many cases of trisomy X are diagnosed before birth via prenatal screening tests such as amniocentesis. Research on females with the disorder finds that cases which were diagnosed postnatally, having been referred for testing because of obvious symptoms, are generally more severe than those diagnosed prenatally. Most females with trisomy X live normal lives, although their socioeconomic status is reduced compared to the general population.

Trisomy X occurs via a process called nondisjunction, in which normal cell division is interrupted and produces gametes with too many or too few chromosomes. Nondisjunction is a random occurrence, and most girls and women with trisomy X have no family histories of chromosome aneuploidy.^{[note 2]} Advanced maternal age is mildly associated with trisomy X. Women with trisomy X can have children of their own, who in most cases do not have an increased risk of chromosome disorders; women with mosaic trisomy X, who have a mix of 46,XX (the typical female karyotype) and 47,XXX cells, may have an increased risk of chromosomally abnormal children.

First reported in 1959 by the geneticist Patricia Jacobs, the early understanding of trisomy X was that of a debilitating disability observed in institutionalized women. Beginning in the 1960s, studies of people with sex chromosome aneuploidies from birth to adulthood found that they are often only mildly affected, fitting in with the general population, and that many never needed the attention of clinicians because of the condition.

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