Genetics Northern California

Trisomy X (Triple X syndrome)

Trisomy X (or triple X syndrome) is a genetic condition that only happens in females. Trisomy X is considered a type of sex chromosome condition, but it does not typically affect female appearance or sexual development. Most girls and women with this condition never know they have an extra X chromosome unless special testing is done.

This condition affects about 1 out of every 1000 girls worldwide.  In California, that means one girl is born with trisomy X almost every day. Girls with this condition look like other girls except they often grow taller than expected compared with their family. Intelligence most often falls in the normal range; but IQ scores tend to be lower than their siblings. Delays in speech, language and motor skills are common in girls with trisomy X. This may contribute to behavior issues, such as shyness and immaturity, especially during school years. Girls with this condition also seem more likely to have low self-esteem and poor self-confidence. However, there is a lot of variability with this condition and it is not possible to predict which girls with trisomy X will have learning or behavior issues.

Cause
Trisomy X is a genetic condition, but it is not inherited from either parent. Genes are the instructions or blueprints that make each human being unique. Our genes are packaged into chromosomes. Humans usually have 46 chromosomes. Chromosomes come in pairs, and we inherit one of each pair from our mother and the other from our father. One pair is the called the sex chromosome pair. These chromosomes include the genetic instructions for a person’s sex.  Normally, females have two X chromosomes, and males have one X and one Y chromosome. Sometimes, instead of having two sex chromosomes, a baby is born with a change in the number of sex chromosomes. Girls and women with trisomy X have three X chromosomes instead of two. This happens completely by chance when an egg or sperm forms.

Diagnosis
Trisomy X can be detected through prenatal tests, such as chorionic villus sampling (CVS) and amniocentesis. This condition can also be diagnosed after birth by chromosome studies, usually done on a blood sample or a cheek swab. The prenatal screening test called non-invasive prenatal testing (NIPT) or cell-free DNA testing can look for pregnancies that may have trisomy X, but more testing is needed to be sure.

Treatment/Services
Girls with trisomy X can benefit from early intervention, such as speech therapy or physical therapy. Treatment for learning disabilities is handled the same as any other child who may need motor, reading, or language help. With early intervention and family support, most girls with trisomy X have performed just as well as girls without this condition. 

RESOURCES
Genetic Home Reference: Triple X syndrome - General information about triple X syndrome provided by the National Library of Health. Also includes links to related sites.

AXYS: http://www.genetic.org/ - A support organization for individuals with X and Y chromosome variations
Brochure on trisomy X (from AXYS):
http://www.genetic.org/Portals/0/Public/Brochures/AXYS%20XXX%20Brochure.pdf

Unique: http://www.rarechromo.org/html/home.asp - A support organization for individuals with rare chromosome conditions.
Brochures on trisomy X (from Unique):
For girls:  http://www.rarechromo.org/information/Chromosome_X/Disclosing_about_XXX_for_girls%20FTNW.pdf
For parents: http://www.rarechromo.org/information/Chromosome_X/Triple_X_syndrome%20Trisomy_X%20FTNW.pdf

Last reviewed: May 13, 2015
Last updated: May 20, 2015
Reviewed by: Regional Genetics Department