AP BIOLOGY

Detailed explanation-1: -That is, the overwhelming majority are due to errors during oogenesis: at least 90% of cases of chromosome 21 nondisjunction are due to maternal meiotic errors [1], [2]. In addition, among these maternal errors, the majority occur during meiosis I (MI) [3], [4].

Detailed explanation-2: -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. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.

Detailed explanation-3: -Mitotic nondisjunction can occur due to the inactivation of either topoisomerase II, condensin, or separase. This will result in 2 aneuploid daughter cells, one with 47 chromosomes (2n+1) and the other with 45 chromosomes (2n-1). Nondisjunction in meiosis I occurs when the tetrads fail to separate during anaphase I.

Detailed explanation-4: -Trisomy 21: About 95% of people with Down syndrome have Trisomy 21. With this type of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.

Detailed explanation-5: -Conditions that arise from non-disjunction events include: Patau’s Syndrome (trisomy 13) Edwards Syndrome (trisomy 18) Down Syndrome (trisomy 21)