PATHOLOGY

Detailed explanation-1: -Nondisjunction of chromosome 21 is the leading cause of Down syndrome. Two risk factors for maternal nondisjunction of chromosome 21 are increased maternal age and altered recombination.

Detailed explanation-2: -Trisomy 21 (nondisjunction) accounts for 95% of known cases of Down syndrome. This type of Down syndrome results in an embryo with three copies of chromosome 21 instead of two and happens when a pair of 21st chromosomes in either the sperm or the egg fails to separate before or during conception.

Detailed explanation-3: -Nondisjunction Examples Nondisjunction causes abnormal number chromosomes in all the cells called aneuploidy or in some cells called mosaicism. 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: -Trisomy 21 or Down syndrome (DS) is one of the most common chromosomal abnormalities. 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: -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.