An abnormal number of chromosomes o

An abnormal number of chromosomes or aneuploidy accounts for most spontaneous abortions, as missegregation of a single chromosome during development is often lethal. Individuals with trisomies of chromosomes 13 or 18, which cause Patau and Edwards syndromes, respectively, are born with severe developmental defects and die soon after birth. Only individuals with trisomy 21, which causes Down syndrome, can live to adulthood. However, they display developmental abnormalities, cognitive disabilities, congenital heart defects, increased risk for leukemias and neurodegenerative disease, autoimmune disorders, and clinical symptoms associated with premature aging. The incidence of aneuploidy is also associated with tumorigenesis and increases with age in both somatic and germline tissues in apparently healthy individuals. The mechanisms by which aneuploidy affects cellular function to cause Down syndrome, promotes tumor formation,or promotes aging are not well understood.