It occurs during the formation of Gametes. So when the gametes are formed in the respective organs in the male and female body, during that time meiosis occurs and the 2N number changes to N, as during fertilization, the two gametes fuse and form the zygote which is also 2n.

If any kind of problem occurs during meiosis then it can lead to a genetic disorder in the offspring. So it is a crucial process in the reproduction process.