|Triple X syndrome|
|Classification and external resources|
Triple X syndrome is a form of 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 and occurs about once in every 1,000 births. Unlike most other chromosomal conditions (such as fragile X), there is usually no distinguishable difference to the naked eye between women with triple X and the rest of the female population.
Triple X syndrome is not inherited, but usually occurs as an event during the formation of reproductive cells (ovum and sperm). An error 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 her cells. In some cases, trisomy X occurs during cell division in early embryonic 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.
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 may have menstrual irregularities, and, although rarely exhibiting severe mental impairments, have an increased risk of learning disabilities, delayed speech, deficient language skills, and delayed development of motor skills.
There are seldom any observable physical anomalies in triple X females, other than being taller than average. Most women with triple X have normal sexual development. Some experience an early onset of menstruation. Triple X women are rarely diagnosed, apart from pre-natal testing methods, such as amniocentesis and blood tests for medical reasons later in life. Most medical professionals do not regard the condition a disability. However, such status can be sought by parents for early intervention treatment if mild delays are present.
The first published report 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.