During the meiosis, the cell is dividing two times and chromosomes are getting replicated. In this process, there can be errors. One of those errors is called nondisjunction.
As a result of the meiosis one, the cell gets divided into two cells and each will carry 23 chromosomes each. At this stage, These chromosomes are connected in the form of sister chromatids. During the meiosis two, these sister chromatids will pull apart and form four cells. Each one will be haploid cells. In meiosis one, chromosomes will undergo crossing over so this separation of chromosomes in the second meiosis is know as disjunction. However, in this process of separation, the chromosomes do not need to get distributed correctly. The errors that happened in this process are known as nondisjunction.
In humans, meiosis will happen and form sperms or eggs( gametes) which are haploid cells and later they combined to form a zygote which is the diploid cell.
