Triple X syndrome

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.

Cause

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.

Symptoms

Due to 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, and language skills.

In Triple X, XYY and Klinefelter's syndrome, a lanky/youthful appearance with increased facial beauty has been described, or in some instances varying degrees of androgeny, but these cases usually reflect traits present in near relatives. An individual producing a child with the above abnormalities has higher than average risk to produce more. Most commonly, there is no observable difference in triple X, other than being taller than average. The additional X chromosome can come from either the maternal or paternal side. The condition is verified only by karyotype testing.

Most women with triple X have normal sexual development accompanied with prolonged physical/emotional youth and are able to conceive children. 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.

Incidence

Triple X syndrome occurs in around 1 in 1,000 girls. On average, five to ten girls with triple X syndrome are born in the United States each day.

First case

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.