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.

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.

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.

[4] Symptoms associated with trisomy X include tall stature, mild developmental delay, subtle physical and skeletal anomalies, increased rates of mental health concerns, and earlier age of menopause.

[3] Other skeletal anomalies associated with trisomy X include clinodactyly (incurved little fingers), radioulnar synostosis (the fusion of the long bones in the forearm),[7] flat feet, and hyper-extensible joints.

[9] Severe internal disease is rare in trisomy X. Genitourinary conditions are more common than in the general population, particularly kidney and ovary malformations.

[12] Conditions such as sleep apnea, asthma, scoliosis, and hip dysplasia have also been linked to sex chromosome aneuploidies as a whole, including trisomy X.

[19] Neuroimaging in trisomy X demonstrates decreased whole brain volumes, correlated with overall intellectual functioning, although cortical thickness is unaffected.

[23] These traits vary in severity; though some women with trisomy X are significantly impaired, many are within the normal range of variance, and some are high-functioning and high-achieving.

[16] Some mental health issues are more frequent in women with trisomy X. Dysthymia and cyclothymia, milder forms of depression and bipolar disorder respectively, are more common than in the general population.

[18] Women with trisomy X are often "late bloomers", experiencing high rates of psychological distress into early adulthood, but by their mid-thirties having stronger interpersonal bonds and healthy relationships.

[23] Although generally milder, 46,XX/47,XXX mosaicism is associated with a higher risk of chromosome anomalies in offspring than full trisomy X.

[3][27] Around 5% of females with Turner syndrome, defined by a karyotype with a single copy of the X chromosome, have a 47,XXX cell line.

Most are still affected by short stature and early premature ovarian failure (before age 30) is common, but a majority reach puberty and menarche spontaneously.

The tetrasomy is generally more severe than the trisomy; intellectual disability is characteristic, dysmorphic features more visible, and puberty often altered.

[26] Proposed mechanisms behind the phenotype of Trisomy X include incomplete X-chromosome inactivation, and corresponding changes to DNA methylation and gene expression across the entire genome.

Around 10% of cases of trisomy X are diagnosed in the person's lifetime; many are ascertained coincidentally during prenatal testing via amniocentesis or chorionic villi sampling, which is routinely performed for advanced maternal age.

[3] Postnatal testing is typically prompted by tall stature,[38] hypotonia, developmental disability, mild dysmorphic features such as hypertelorism or clinodactyly, and premature ovarian failure.

The phenotype of pentasomy X is more severe than the trisomy or tetrasomy, with significant intellectual disability, heart defects, microcephaly, and short stature.

[45] Large cytogenetic studies in Denmark find a diagnosed prevalence of 6 in 100,000 females, around 7% of the actual number of girls and women with trisomy X expected to exist in the general population.

[42] Amongst the 244,000 women in the UK Biobank research sample, 110 were found to have 47,XXX karyotypes, corresponding to approximately half the number expected in the population.

[12] The first known case of trisomy X, in a 176 cm (5 ft 9+1⁄2 in) woman who experienced premature ovarian failure at the age of 19, was diagnosed in 1959 by a team led by Patricia Jacobs.

[5][51] The late 1950s and early 1960s were a period of frequent ascertainment of previously unknown sex chromosome aneuploidies, with the 47,XXX karyotype discovered alongside 45,X and 47,XXY the same year.

[55] In response to the biased early studies, a newborn screening program for sex chromosome aneuploidy disorders was implemented in the 1960s.

[5] The children with trisomy X and Klinefelter's had their karyotypes disclosed to their parents, but due to the then-present perception that XYY syndrome was associated with violent criminality, those diagnoses were hidden from the family.

[56] These studies dispelled the idea that sex chromosome aneuploidies were "tantamount to a life of serious handicaps" and revealed their high prevalence in the population.

[57] They provided extensive information on the outcomes of trisomy X and other sex chromosome aneuploidies, forming much of the medical literature on the topic to this day.

[68] Three of the six known cases of canine trisomy X demonstrated behavioural issues such as fearfulness, inciting speculation about a link between the karyotype and psychological concerns as seen in humans with the condition.

An additional dog with normal fertility and no reported behavioural issues was found to have a mosaic 78,XX/79,XXX karyotype.