monosomy
Monosomy is having only one copy of a chromosome that should come as a pair — the missing twin on a shelf built for two. With just a single copy, the cell has only half the normal dose of every gene on that chromosome, and for most chromosomes that shortfall is even harder to tolerate than having an extra one.
Monosomy is a form of aneuploidy and arises the same way as trisomy: nondisjunction produces a gamete missing a chromosome, so the embryo ends up with one copy instead of two. Because losing genes outright is generally worse than over-dosing them, autosomal monosomies almost always cause miscarriage and essentially never survive to birth.
The one notable exception is monosomy of the X chromosome, written 45,X, which causes Turner syndrome. It is survivable largely because the body normally silences a second X anyway through X-inactivation, so a single active X is part of the cell's usual working state. Even so, many 45,X conceptions still end in early pregnancy loss.
A 45,X karyotype — a single X and no second sex chromosome — is monosomy X, the chromosomal basis of Turner syndrome.
Monosomy X is survivable; autosomal monosomies essentially are not.
Losing a chromosome (monosomy) is usually less survivable than gaining one (trisomy); Turner syndrome is the rare human monosomy compatible with birth.