This is failure of chromosomes pairs to separate properly during meiosis is?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis.[1][2][3] Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

Aneuploidy is type of genetic disorder. It can be defined as the non disjunction which takes place when the pair of the sister chromatids or homologous chromosomes fail to get separate during the time of meiosis.

When a single chromosome is lost from the diploid condition, a new condition arises known as monosomy. The monosomy condition can be represented as 2n-1 numerically.

Whereas the gain of one chromosome is known as 2n+1. This can cause defect in the human body.