In almost all cases, it is not passed down through families. Instead, it is the result of a single faulty egg or sperm. Persons with Down syndrome are mentally disabled. They have somewhat flat facial features and slant ed ing eyes.
Many affected persons have heart defects and other disorders.
The chance of having a baby with Down syndrome rises as parents get older. It depends more on the mother's age. The chance goes up quickly when the mother is over 34 years. It also goes up when the father is over 60 years of age. This suggests that the main cause of the disorder is the aging of egg cells or lifelong exposures, such as radiation or viruses.