monosomies

Monosomy is a chromosomal abnormality in which a cell has only one copy of a particular chromosome instead of the usual two. In humans, complete monosomies of autosomes are typically lethal before birth, and very few affected pregnancies result in live birth. The most well-known viable monosomy is monosomy X, which produces Turner syndrome, usually described as 45,X, though mosaic forms such as 45,X/46,XX can also occur.

In Turner syndrome, individuals have a single X chromosome and lack an additional sex chromosome. Mosaic monosomies involving the sex chromosomes (for example, 45,X/46,XY) can occur and may lead to a range of phenotypes from mild to more typical Turner-like features. Monosomies of autosomes (for example, missing one copy of chromosome 1 or 7) are generally not compatible with long-term survival, and their occurrence is typically identified only in pregnancy or in very rare postnatal cases as part of mosaic conditions.

Causes of monosomy include nondisjunction during meiosis, leading to gametes with a missing chromosome, or mitotic errors after fertilization that create a monosomic cell line (mosaicism). The clinical presentation often depends on whether the monosomy is complete for a chromosome or limited to a mosaic subset of cells.

Diagnosis is usually by karyotyping from blood or other tissues, with additional methods such as chromosomal microarray or fluorescent in situ hybridization used to characterize mosaicism. Management is supportive and multidisciplinary, focusing on the specific features present, such as growth hormone therapy and estrogen replacement in Turner syndrome, cardiovascular monitoring, and psychosocial support. There is no cure for monosomy itself.