Turner's syndrome is a human female disorder characterized by short stature, obesity, and failure to mature sexually. It is caused by a missing X chromosome, so the affected person has only 45 chromosomes. When there is an extra or missing chromosome, the chromosomes never separate from the tetrad during metaphase of meiosis I. They are always XO, as YO is a fatal condition prior to birth. How can a genetic disorder, such as Turner's syndrome, occur?