AP BIOLOGY

Detailed explanation-1: -About 95 percent of the time, Down syndrome is caused by trisomy 21-the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.

Detailed explanation-2: -Trisomy 21 (Nondisjunction) Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two.

Detailed explanation-3: -The overwhelming majority of trisomy 21, or Down syndrome, is caused by the failure of chromosomes to separate properly during meiosis, also known as chromosome nondisjunction.

Detailed explanation-4: -The majority of full trisomy 21 is caused by chromosomal nondisjunction occurring during maternal meiotic division (∼90%).