PATHOLOGY

Detailed explanation-1: -If non-disjunction occurs, it can result in individuals having an abnormal number of chromosomes (for example, three copies of chromosome 21 ) in their cells. Non-disjunction is responsible for 95 per cent of Down’s syndrome cases.

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: -Patients with Down syndrome have an extra copy of their 21st chromosome. This extra chromosome is usually acquired before fertilization in meiosis, which is the process where sperm and egg cells, or gametes, are made.

Detailed explanation-4: -Nondisjunction can occur during either meiosis I or II, with differing results. If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.

Detailed explanation-5: -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).