Triple X syndrome is a chromosomal variation characterized 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, and their weight may be low in comparison to their height. They 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, males with an additional X chromosome). These include a lanky, youthful appearance, non-affectedness, or having varying degrees of androgyny such as 'huskiness' (triple X) and male breast tissue (XXY). Because such a variety in the condition exists, further categorization 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 estrogen 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 chromosome. The additional X chromosome 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.
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 1 in 1000 newborn girls. Five to ten girls with triple X syndrome are born in the United States each day.
The first published report (Jacobs et al. 1959) of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland in 1959. It was found in a 35-year-old, 5 ft. 9 in. [176 cm] tall, 128 lb. [58.2 kg] woman who had premature ovarian failure at age 19; her mother was age 41 and her father was 40 at the time of her conception.
- Jacobs PA, Baikie AG, Brown WM, MacGregor TN, Maclean N, Harnden DG. "Evidence for the existence of the human 'super female'". Lancet 1959 Sep 26;2:423-5. PMID 14406377
- Klinefelter Syndrome & Associates (http://www.genetic.org/)
- Triple X syndrome at the NLM Genetics Home Reference site
- Booklet from the Turner Center in Denmark
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