NEET BIOLOGY

Detailed explanation-1: -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-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 majority of full trisomy 21 is caused by chromosomal nondisjunction occurring during maternal meiotic division (∼90%).

Detailed explanation-4: -If nondisjunction occurs in meiosis II, two of the four products will be unaffected by the event and two of the products will be abnormal. One abnormal product will have an extra chromosome, and the other abnormal product will be missing that chromosome.

Detailed explanation-5: -There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis.