AP BIOLOGY

Detailed explanation-1: -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-2: -In Nondisjunction Trisomy 21, the most typical type of Down syndrome, there is a failure of the chromosome 21 pair to disjoin from each other or divide properly in the egg or sperm cells, leaving an extra number-21 chromosome in each cell. Trisomy 21 accounts for 95% of Down syndrome cases.

Detailed explanation-3: -Nondisjunction Examples Some of the important examples are: Down’s syndrome – Trisomy of autosomes, i.e. chromosome 21. It contains one extra chromosome 21.

Detailed explanation-4: -The majority of full trisomy 21 is caused by chromosomal nondisjunction occurring during maternal meiotic division (∼90%). Errors occur more frequently in the first maternal meiotic division than the second (73% vs.

Detailed explanation-5: -The majority of numerical chromosomal aberrations, such as Down syndrome, result from a nondisjunction error in meiosis. A daughter cell that has received a pair of a specific parental chromosome will result in a proband trisomic for that chromosome once conception has occurred.