Aneuploidy, the occurrence of abnormal numbers of chromosomes within a cell, including loss of chromosomes (e.g. loss of X chromosome in XO females with Turner's syndrome) or the presence of additional chromosome material as in Down syndrome (where a complete additional copy of chromosome 21 is found). The presence of three copies of chromosomes, instead of the normal complement of two, is called trisomy. The addition or loss of chromosomal material is often very deleterious to life. The gene density on chromosomes X, Y, 13, 18 and 21 is the lowest of all the chromosomes, helping to explain why gross numerical abnormalities on these chromosomes can be tolerated by the growing fetus. Aneuploidy of the other chromosomes almost certainly occurs and is probably incompatible with life, resulting in spontaneous abortion of the developing fetus. Studies have described that a majority of spontaneous abortions have such gross genetic defects.

Trisomy is a common cause of birth defects and mental disabilities. Trisomy 21 occurs at a frequency of about 1:700 live births (see Table). The frequency of the other defects (trisomy 13 or 18) is considerably lower. Other numerical chromosome disorders occur at a low frequency, e.g. XXYY, XXXY, XXXX, etc. These are also amenable to investigation by STR-PCR analysis