Triple X syndrome (Super female, XXX) occurs when a female has three X chromosomes (47, XXX) instead of the normal 46, XX. It results from non-disjunction during meiosis. Most affected individuals are phenotypically female with mild learning difficulties or are clinically normal. Down syndrome = trisomy 21; Klinefelter's syndrome = 47, XXY (male with extra X); Turner's syndrome = 45, X0 (missing one X chromosome, female, short stature, gonadal dysgenesis).