Trisomy is a cell that has three copies of one chromosome instead of having the normal two copies. Trisomy 21 is known as the congenital disorder Down syndrome. This occurs when each cell has three copies of chromosome twenty one rather than having a pair. Commonly, Down syndrome is caused by nondisjunction. Nondisjunction is "an error in which homologous chromosomes or sister chromatids fail to separate normally during meiosis or mitosis" (Huether,McCance pp.42). Scientists have discovered 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. This risk factor increases significantly after the age of 35 for mothers. The older
