Another sex chromosome abnormality is Triple X syndrome (and no it has nothing to do with the movie) Usually, human males have an X and a Y chromosome, females have two X's. Girls with Triple X have , obviously enough, three copies of the X chromosome. It occurs by chance and is seen in about 1 in 1000 females.

Girls with Triple X don't look different, although they are usually taller, nor do they have fertility problems. They are at increased risk for learning delays and mental illness. On average, their IQ is about 11 points lower than siblings.