SEX CHROMOSOME ABNORMALITIES
Chromosome abnormalities arise several different ways. If the sex chromosomes fail to separate correctly during meiosis (chromosome nondisjunction), this can lead to an egg cell or sperm with two X's, two Y's, or with neither an X or a Y. Fertilization involving one of these abnormal gametes results in a zygote with either an extra or missing chromosome (aneuploidy). Aneuploidy causes various clinical syndromes. Sex chromosome aneuploidies include Klinefelter syndrome (XXY), Turner syndrome (XO), triple-X (XXX), and the XYY karyotype. Individuals with Klinefelter syndrome have 47 chromosomes, are sterile, and may show some degree of mental retardation. Individuals with Turner syndrome (XO) are sterile and have underdeveloped genital structures.

Humans can tolerate sex chromosome aneuploidies better than other aneuploidies because of dosage compensation. Dosage compensation refers to the way the system inactivates one X chromosome in normal females. As a result of dosage compensation, the expression of X-linked genes is equivalent in normal females with two X chromosomes and normal males with one X chromosome.