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Triple-X syndrome

Triple X syndrome is a chromosomal variation characterised by the presence of an extra X chromosome in each cell of a human female. The condition is also known as triplo-X, trisomy X, XXX syndrome, and 47,XXX aneuploidy. Triple X results during division of a parent’s reproductive cells. Females with the condition are not at any increased risk for medical problems.


Due to the lyonization, inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell. Thus, triple X syndrome most often causes no unusual physical features or medical problems. Females with the condition are usually taller than average, may have menstrual irregularities, and, although rarely exhibiting severe mental impairments, sometimes have an increased risk of learning disabilities and delayed speech and language skills.

Similar body types and characteristics are present in both triple X and Klinefelter’s syndrome (or XXY: the male equivalent of triple X, males with an additional X chromosone). These include a lanky, youthful appearance, non-affectedness, or having varying degrees of androgeny such as ‘huskiness’ (triple X) and male breast tissue (XXY). Because such a variety in the condition exists, further catagorization may prove beneficial. No research has been done to explain the cause for different body types (whether, for instance, this is simply the effects of the additional X on oestrogen and progesterone levels). It is also uncertain whether family history plays any role in determining how the additional X, once present, is expressed. Body types/characteristics often follow traits shown in family members who are unaffected. Also, mothers of triple X girls frequently identify with and bear many of their child’s traits without themselves having the additional chromosone. The additional X chromosone can come from either the maternal or paternal side. Although body types and characteristics are distinguishable in triple X, the condition is verified only by karyotype testing.

Most women with triple X have normal sexual development and are able to conceive children. A few may experience early onset of menstruation. It has not been determined yet as to whether the latter could be due to increases in both estrogen and progesterone, which work together in female sexual development.

A vast majority of the females with triple X have never been diagnosed. The usual diagnosis results from pre-natal testing methods, such as amniocentesis. Most medical professionals do not regard Triple X syndrome a disability, however disability status can be sought by parents for early intervention treatment when mild delay is present.


Triple X syndrome is not inherited, but usually occurs as a random event during the formation of reproductive cells (ovum and sperm). A change in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the nondisjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of his or her cells. In some cases, trisomy X occurs during cell division in early fetal development.

Some females with triple X syndrome have an extra X chromosome in only some of their cells. These cases are called 46,XX/47,XXX mosaics.


Triple X syndrome occurs in about 1 in 1,000 newborn girls. Five to ten girls with triple X syndrome are born in the United States each day.

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