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Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>People normally have 46 chromosomes in each cell. Two of the 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics. Females typically have two X chromosomes (46,XX), and males typically have one X chromosome and one Y chromosome (46,XY).</p><p>Trisomy X results from an extra copy of the X chromosome in each of a female&#x27;s cells. As a result of the extra X chromosome, each cell has a total of 47 chromosomes (47,XXX) instead of the usual 46.</p><p>Some females with trisomy X have an extra X chromosome in only some of their cells. This phenomenon is called 46,XX/47,XXX mosaicism. Individuals with this form of the condition typically have milder signs and symptoms than those with trisomy X in all of their cells.</p><p>Early in the embryonic development of females, one of the X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation. X-inactivation ensures that females have only one functional copy of the X chromosome in each cell.</p><p>Some genes on the X chromosome do not undergo X-inactivation. Many of these genes are located at the ends of the X chromosome and are also present on the Y chromosome. As a result, males and females each have two functional copies of these genes. Many of these genes are essential for normal development. It is likely that the extra copies of these genes contributes to the signs and symptoms of trisomy X.</p><p>Additionally, variations in the pattern of X-inactivation between individuals and even among cells in a single individual probably play a large role in the range of features seen in people with trisomy X.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/trisomy-x/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Trisomy X?"},{"bodyHtml":"<p>Trisomy X is associated with an increased risk of learning disabilities and delayed development of speech and language skills. Delayed development of motor skills (such as sitting and walking), weak muscle tone (hypotonia), and behavioral and emotional difficulties are also possible, but these characteristics vary widely. Seizures or kidney abnormalities occur in about 10 percent of affected females.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/trisomy-x\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"trisomy-x","sourceDocuments":[{"name":"Trisomy X","reviewed":"2022-02","sections":{"causes":{"fetchedAt":"2026-10-20T20:25:34.493750+00:00","label":"Causes","pageSha256":"cdbcb8dc4bc4cdebb701b105a82f1f5adfa5455a9eaf3da7dd11534c7ed0ce48","sourceUrl":"https://medlineplus.gov/genetics/condition/trisomy-x/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/trisomy-x"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Trisomy X"},"schemaVersion":1}