NEET BIOLOGY

Detailed explanation-1: -Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases result from nondisjunction during maternal meiosis I.

Detailed explanation-2: -Nondisjunction during meiosis is the most common cause of the aneuploidies, which can also occasionally result from a chromosomal rearrangement (e.g., Robertsonian translocation or a balanced sex chromosome translocation) [26].

Detailed explanation-3: -Nondisjunction occurs when chromosomes fail to segregate during meiosis; when this happens, gametes with an abnormal number of chromosomes are produced. The clinical significance is high: nondisjunction is the leading cause of pregnancy loss and birth defects.

Detailed explanation-4: -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-5: -Mitotic nondisjunction can cause somatic mosaicism, with the chromosome imbalance only reflected in the direct offspring of the original cell where the nondisjunction occurred. This can cause some forms of cancer, including retinoblastoma.