CausesBy Mayo Clinic staff
Most people have 46 chromosomes occurring in 22 pairs, plus two sex chromosomes, one maternal and one paternal. These chromosomes contain genes, which carry instructions that determine everything from your height to your eye color.
One of these chromosome pairs determines your sex. You receive one sex chromosome from your mother and another from your father. Your mother can give you only an X chromosome, but your father can pass on an X or a Y chromosome. If you receive an X chromosome from your father, the XX pair makes you genetically a female. If you receive a Y chromosome from your father, then your XY pair means you're genetically a male.
Females with triple X syndrome have a third X chromosome. Although this condition is genetic, it's typically not inherited. Instead, what usually happens is that either the mother's egg cell or the father's sperm cell has not formed correctly, resulting in an extra X chromosome. This random error in egg or sperm cell division is called nondisjunction.
When the cause is a malformed egg cell or sperm cell, as is usually the case, all the cells in the offspring's body have the extra chromosome. Occasionally, the extra chromosome doesn't appear until early in the development of the embryo. If this is the case, then the female is said to have a mosaic form of triple X syndrome.
In the mosaic form, only some of the body's cells have the third X chromosome. Because only some cells contain the extra X chromosome, females with the mosaic form of triple X syndrome may have less severe symptoms.
Triple X syndrome is also called trisomy X, triplo X syndrome and XXX syndrome. It's also referred to as 47,XXX syndrome because of the existence of a 47th chromosome, which is the extra X chromosome.
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