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Trisomy X

(Triple X Syndrome; XXX)

By

Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jul 2020| Content last modified Jul 2020
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The sex chromosomes Sex chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells... read more Sex chromosomes determine whether a fetus becomes male or female. A pair of X and Y chromosomes (XY) results in a male, and a pair of X and X chromosomes (XX) results in a female.

In trisomy X, the extra X chromosome is usually inherited from the mother. The older the mother is, the greater the chance her fetus could have the syndrome. About 1 in every 1,000 girls is born with the third X chromosome.

Trisomy X rarely causes obvious physical abnormalities. Girls with trisomy X may have slightly lower intelligence, problems with verbal skills, and more school problems than siblings. Sometimes the syndrome causes menstrual irregularities and infertility. However, some women with trisomy X have given birth to physically normal children who have normal chromosomes.

Extremely rare cases of infants with four or even five X chromosomes have been identified. The more X chromosomes the girl has, the greater the chance of intellectual disability Intellectual Disability Intellectual disability is significantly below average intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily... read more and physical abnormalities.

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