True about trisomies is -

Trisomy 21 or Down syndrome is the most common of the chromosomal disorder. The most common cause of trisomy and therefore of Down syndrome is meiotic nondisjunction (type II Meiosis error). a Maternal age has a strong influence on the incidence of trisomy 21. It occurs once in 1550 live births in women under age 20 in contrast to 1 in 25 live births for mothers over age 45. The correlation with maternal age suggests that in most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum. Approximately 40% of the patients with trisomies have associated congenital heart disease, most common defects of the endocardial cushion including. Ostium premium atrial septal defects. A-V valve malformations and VSDS Down syndrome is an important cause of mental retardation in children