CHROMOSOME ANEUPLOIDY: AUTOSOMES
Most autosomal aneuploidies have devastating effects on the fetus and end in miscarriage. A common syndrome resulting from nondisjunction of autosomes (chromosomes other than sex chromosomes) in which many children survive is Down syndrome. Individuals with Down syndrome often have 47 chromosomes (instead of the usual 46). Most are trisomic for chromosome 21, meaning that they have three copies of that chromosome. Extra copies of autosomes cause genetic imbalance. In Down syndrome, this results in abnormal physical development (face, eyelids, tongue, hands) and, frequently, mental retardation.

Because Down syndrome is much more common in children of older mothers, at-risk pregnant women regularly choose amniocentesis and karyotyping of the fetal fluid to determine whether the fetus has a normal karyotype or an extra chromosome 21.