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: -Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21. The type of error is called nondisjunction (pronounced non-dis-JUHNGK-shuhn).

Detailed explanation-4: -Answer and Explanation: Down syndrome is a genetic disorder in which a diploid cell consists of three copies of autosomal chromosomes (other than X and Y chromosomes) instead of two. This is referred to as autosomal trisomy. This condition arises due to the nondisjunction during meiosis I.

Detailed explanation-5: -Trisomy 21 is the most common chromosomal anomaly in humans, affecting about 5, 000 babies born each year and more than 350, 000 people in the United States. Also known as Down syndrome, trisomy 21 is a genetic condition caused by an extra chromosome.