Trisomy is a cell that has three copies of one chromosome instead of having the normal two copies. Trisomy 21 is the congenital disorder better known as Down syndrome. This occurs when each cell has three copies of chromosome twenty one rather than having a normal pair. Nondisjunction is "an error in which homologous chromosomes or sister chromatids fail to separate normally during meiosis or mitosis" (Huether,McCance pp.42). Therefore, nondisjunction is a common cause of Down syndrome. Scientists have made the discovery that Down syndrome is caused by an extra chromosome, but the cause of why that occurs is unknown. The main risk for having a child with Down syndrome is maternal age. After the mother reaches the age of 35, the risk of
