X-Linked recessive trait

XXX syndrome ( Triple X syndrome, trisomy X)

A condition characterized by the presence of three X chromosomes in each of a female's cells, instead of the usual two X chromosomes. Although females with triple X tend to be slightly taller than average the effect of having an extra X chromosome appears to be otherwise quite variable. Many females will show no or few symptoms while others may have decreased fertility, learning difficulties, and developmental delay and/or behavioral problems. 

XXX syndrome occurs in one out of 1000 female births, and appears to be the result of an extra X chromosome contributed by the mother's ovum (egg). The error occurs during the production of the mother's eggs when the X chromosomes fail to separate and migrate into separate cells.  This error in the distribution of chromosomes during the formation of reproductive cells is called nondisjunction and occurs more frequently in older mothers.