Question

What is nondisjunction?

Answer 1

Hint: Aneuploidy is the presence of an aberrant number of chromosomes in a cell, such as 45 or 47 instead of the usual 46 in a human cell. A difference of one or more entire sets of chromosomes is not included. A euploid cell is one that has any number of full chromosomal sets.
Some genetic abnormalities are caused by an extra or missing chromosome. Atypical chromosome counts can also be found in cancer cells.

Complete answer:
The inability of chromosomes or chromatids to segregate during cell division is known as nondisjunction. Aneuploidy is a condition in which daughter cells have an abnormal number of chromosomes. During cell division, the irregular distribution of chromosomes causes one cell to have an extra chromosome and the other to have a chromosomal deficiency.
Nondisjunction is divided into two types: mitotic nondisjunction and meiotic nondisjunction.
Mitotic Sister chromatids fail to split during anaphase, resulting in nondisjunction.
There are two types of meiotic nondisjunction. In the first kind, homologous chromosomes fail to segregate during anaphase I due to nondisjunction during meiosis I, resulting in all haploid cells having an incorrect number of chromosomes. When sister chromatids fail to segregate during meiosis II, the second form of nondisjunction occurs. Half of the haploid cells have defective chromosomes as a result.
Inactivation of topoisomerase II, separase, or condensin causes nondisjunction. Separase breaks the cohesin that holds the sister chromatids together during anaphase. Condensin and topoisomerase II remove catenation.
Because the oocyte meiotic division is inhibited initially at diplotene of prophase I and then later at metaphase II, nondisjunction is more likely in oocytes. The mother is responsible for the majority of aneuploidy in children.

Note:
Examples of nondisjunction-
1. Down's syndrome is caused by chromosome $$21$$'s trisomy of autosomes.
2. Edwards syndrome is caused by chromosome $$18$$ trisomy.
3. Patau syndrome is caused by chromosome $$13$$ trisomy.
4. Trisomy of sex chromosomes is known as Klinefelter syndrome.
5. Monosomy is a symptom of Turner syndrome. One of the X chromosomes has been lost.