aneuploidy

Aneuploidy is an abnormal number of chromosomes in a cell that is not an exact multiple of the haploid set. In humans, euploid cells carry 46 chromosomes; aneuploid cells carry 45, 47, or another abnormal number due to the gain or loss of individual chromosomes. Aneuploidy most commonly arises from nondisjunction during meiosis in the parental gametes or from mitotic errors after fertilization, and it can be restricted to certain tissues in mosaic forms.

Survival with autosomal aneuploidy is rare. The most familiar viable autosomal trisomies are trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome), each with distinct clinical features and typically limited lifespan. Sex chromosome aneuploidies—such as Turner syndrome (45,X), Klinefelter syndrome (47,XXY), Triple X (47,XXX), and XYY—are more common and show a wide spectrum of phenotypes.

Mosaic aneuploidy arises when nondisjunction occurs after fertilization, producing two or more cell lines with different chromosome complements. This can moderate severity and alter clinical outcomes depending on the proportion and distribution of abnormal cells across tissues. Other mechanisms, including unbalanced translocations in a parent, can also generate aneuploid offspring.

Diagnosis and detection include karyotyping, fluorescence in situ hybridization, chromosomal microarray, and noninvasive prenatal testing using cell-free DNA. Postnatal diagnosis relies on cytogenetic testing of blood or tissue. The incidence of aneuploidy in conceptions is high, but most affected embryos are not viable. Management is focused on associated medical needs and developmental support.