Monosomy can occur due to errors during meiosis, the process of cell division that produces gametes (sperm and egg cells). Specifically, it results from nondisjunction, where chromosomes fail to separate properly. This leads to one gamete containing two copies of a chromosome and the other gamete containing none. When a gamete with no copy of a chromosome fuses with a normal gamete, the resulting zygote will have monosomy for that chromosome.
