What are chromosomal disorders?

Chromosomal disorders occur when children do not have the correct pairings or complement of 46 chromosomes. Chromosomal abnormalities are more common in children of older mothers and fathers. Down syndrome, for example, is caused by having an extra chromosome on the 21st pair, resulting in 47 chromosomes. There are also disorders linked to the sex chromosomes. For example, "supermales" have an extra Y chromosome on the 23rd pair. Males with an extra X chromosome are said to have Klinefelter syndrome, characterized by underdeveloped male secondary sex characteristics and mild mental retardation. A female with a single X chromosome is said to have Turner syndrome, characterized by underdevelopment of female secondary sex characteristics and problems in mathematics and visual-spatial skills.