monossomias

Monossomias is a term used in medical genetics to describe chromosomal abnormalities in which one chromosome is missing from the normal diploid set in a subset of cells, resulting in mosaic monosomy. The condition is typically identified when a tissue sample shows two cell lines, one with the standard chromosome complement and another with a monosomy for one chromosome. Most autosomal monosomies are not compatible with life when present in all cells, so viable cases generally involve mosaicism or monosomy of the sex chromosomes.

Causes and mechanism often involve nondisjunction during meiosis with subsequent loss of a chromosome during early mitotic divisions after fertilization. This creates two or more cell lines within the same individual, leading to variable expression depending on the proportion and distribution of monosomic cells across tissues.

Diagnosis is established through cytogenetic and molecular testing. Conventional karyotyping, fluorescence in situ hybridization (FISH), and chromosomal microarray analyses can detect mosaic monosomies in blood, skin fibroblasts, or other tissues. Prenatal detection is possible via chorionic villus sampling or amniocentesis, and noninvasive prenatal testing may reveal sex-chromosome monosomies or certain autosomal monosomies depending on the platform and probe set used.

Clinical features vary with the chromosome involved and the degree of mosaicism. When monosomy involves the sex chromosomes, conditions such as Turner syndrome (45,X) are well described and may be compatible with life in mosaic forms. Autosomal monosomies are associated with more severe developmental and congenital anomalies and are often identified in prenatal or perinatal settings. Management is supportive and multidisciplinary, focusing on the specific anomalies present, growth and development monitoring, and treatment of associated conditions.

Etymology derives from Greek monos, “one,” and soma, “body.”