Fragile x syndrome

[Marquetta]

Fragile X syndrome is a genetic abnormality on the X chromosome that leads to intellectual disability and behavior problems.
Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.
Fragile X syndrome is the most commonly diagnosed inherited cause of intellectual disability, and boys are affected more often than girls. Fragile X syndrome is second only to Down syndrome as a cause of intellectual disability in boys. However, unlike Fragile X syndrome, Down syndrome is not inherited in most cases. For more information, see the National Fragile X Foundation.
The symptoms of Fragile X syndrome are caused by an abnormality of a specific gene on the X chromosome (see Gene abnormalities). People have an excessive number of copies (more than 200) of a small segment of deoxyribonucleic acid (DNA). People with 55 to 200 extra copies are considered to have a premutation because, although they do not have the disorder, their offspring are at greater risk of having it.