A genetic abnormality in which a diploid organism is missing one copy of one of its chromosomes. Normally, diploid organisms have two copies of each chromosome in their genome. An individual with monosomy has has only one copy of one of its chromosomes.

Monosomy occurs when one of the haploid gametes that fuse to form a zygote is missing a chromosome. Like trisomy, a gamete can end up without on of its chromosomes due to nondisjunction at the first division of meiosis, when a tetrad fails to separate and both homologous chromosomes go into the same daughter cell. One daughter cell has two (and can go on to produce a gamete that can result in trisomy), while the other daughter cell is left without, and can go on to produce a gamete that will result in monosomy. Also, nondisjunction at the second meiotic division (when a pair of chromatids fail to separate, and both go to the same side) can also result in the other gamete missing a chromosome.

Monosomy often results in an individual that can't survive. Some types of monosomy, however, aren't so bad, and the individual can survive. One fairly common example in humans is Turner's Syndrome, in which the individual is monosomic for the X chromosome.