monosomy

Monosomy is a chromosomal abnormality in which a cell has only one copy of a chromosome instead of the usual two in a diploid organism. In humans, monosomy for autosomes is typically lethal during embryonic development. The most common and only widely observed viable monosomy is monosomy X, which results in Turner syndrome. Monosomy can also occur in mosaic form, where some cells have the monosomy and others do not.

Causes of monosomy include nondisjunction during meiosis or mitosis, leading to a zygote missing a chromosome, or loss of a chromosome in early cell divisions after conception. Mosaic monosomy arises from postzygotic chromosomal loss. True monosomy of an entire autosome generally does not produce a viable live birth, though mosaic cases may survive with varying effects depending on the tissues involved.

Clinical features depend on which chromosome is affected. Turner syndrome (45,X or mosaic 45,X/46,XX) commonly presents with short stature, gonadal dysgenesis, primary or delayed puberty, lymphedema, a webbed neck, and cardiovascular or renal anomalies. Monosomies of other autosomes are usually not compatible with life, often leading to miscarriage; when mosaicism is present, survival and phenotype reflect the proportion and distribution of monosomic cells.

Diagnosis is usually made by karyotyping of blood or fetal cells. Additional methods such as FISH, comparative genomic hybridization, or SNP arrays can detect monosomy and mosaicism, especially in prenatal testing.

Management focuses on features and associated conditions: growth hormone therapy and pubertal induction in Turner syndrome; monitoring cardiovascular, renal, and other potential issues; counseling on fertility options. Prognosis varies with the specific chromosome involved and the degree of mosaicism.