AP BIOLOGY

Detailed explanation-1: -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-2: -Trisomy 21 (Nondisjunction) Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.

Detailed explanation-3: -Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy.’ Down syndrome is also referred to as Trisomy 21.

Detailed explanation-4: -Trisomy 21 in humans, commonly referred as Down syndrome (DS), is the most common genetic cause of mental retardation. In approximately 95% cases, the extra chromosome occurs as a result of meiotic nondisjunction (NDJ) or abnormal segregation of chromosomes.

Detailed explanation-5: -Nondisjunction means that a pair of homologous chromosomes has failed to separate or segregate at anaphase so that both chromosomes of the pair pass to the same daughter cell. This probably occurs most commonly in meiosis, but it may occur in mitosis to produce a mosaic individual.