AP BIOLOGY

Detailed explanation-1: -The abnormality usually occurs in egg cells, but it occasionally occurs in sperm cells. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21.

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: -In Down syndrome, there is an additional copy of chromosome 21, resulting in three copies instead of the normal two copies. Down syndrome is a genetic disorder caused when abnormal cell division results in an extra full or partial copy of chromosome 21.

Detailed explanation-4: -The extra X chromosome is retained because of a nondisjunction event during meiosis I (gametogenesis). Nondisjunction occurs when homologous chromosomes, in this case the X and Y sex chromosomes, fail to separate, producing a sperm with an X and a Y chromosome. Fertilizing a normal (X) egg produces an XXY offspring.

Detailed explanation-5: -However, when nondisjunction occurs, the chromatids do not separate. The result is that one cell receives both chromatids, while the other cell receives neither. Each daughter cell then has an abnormal number of chromosomes when mitosis is complete; one cell has an extra chromosome, while the other is missing one.