Trisomy X, also called triple X syndrome or 47,XXX, is characterized by the presence of an additional X chromosome in each of a female's cells. Although females with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most females with trisomy X have normal sexual development and are able to conceive children.

People normally have 46 chromosomes in each cell. Two of the 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics. Females typically have two X chromosomes (46,XX), and males have one X chromosome and one Y chromosome (46,XY).

This condition occurs in about 1 in 1,000 female newborns; however, many of these affected individuals are never diagnosed.  Five to 10 people with trisomy X are born in the United States each day.

Most cases of trisomy X are not inherited. The chromosomal change usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of the X chromosome as a result of nondisjunction. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of the body's cells.

Published Date: February 28, 2022
Published By: National Institutes of Health