aneuploidies

An aneuploidy is an abnormal number of chromosomes in a cell, meaning the chromosome number is not an exact multiple of the haploid set. In humans, aneuploidies most often arise from nondisjunction during meiosis or mitosis, yielding gametes or zygotes with extra or missing chromosomes. Many aneuploid conceptuses fail to develop, but some survive to birth with characteristic syndromes. Aneuploidies can involve autosomes or sex chromosomes; by far most fetal autosomal aneuploidies are lethal, but three autosomal trisomies are compatible with life: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Features vary but commonly include intellectual disability, congenital anomalies, and growth issues, with severity highest in trisomy 18 and 13.

Sex chromosome aneuploidies include Turner syndrome (monosomy X), Klinefelter syndrome (XXY), trisomy X (XXX), and XYY syndrome; presentations range from mild to noticeable developmental and reproductive differences. Mosaic aneuploidy occurs when some cells are aneuploid while others are normal, due to postzygotic mitotic errors.

Diagnosis involves karyotyping, fluorescence in situ hybridization, or chromosomal microarray; prenatal detection uses noninvasive prenatal testing of cell-free DNA, as well as diagnostic tests such as chorionic villus sampling or amniocentesis.

Management is supportive and multidisciplinary, focusing on medical issues, developmental therapies, and reproductive counseling. There is no cure for aneuploidy; risk is influenced by parental age and other factors, and outcomes depend on the specific chromosome involved and the degree of mosaicism.