Chromosomal Aberrations

Aneuploidy is the gain or loss of individual chromosomes from the normal diploid set of forty-six chromosomes. As in structural anomalies, the error may be present in all cells of a person or in a percentage of cells. Changes in chromosome number generally have an even greater effect upon survival than changes in chromosome structure. Considered the most common type of clinically significant chromosome abnormality, it is always associated with physical and/or mental developmental problems. Most aneuploid patients have a trisomy of a particular chromosome. Monosomy, or the loss of a chromosome, is rarely seen in live births. The vast majority of monosomic embryos and fetuses are probably lost to spontaneous abortion during the very early stages of pregnancy. An exception is the loss of an X chromosome, which produces Turner's syndrome. Trisomy may exist for any chromosome, but is rarely compatible with life.

Aneuploidy is believed to arise from a process called nondisjunction. Nondisjunction occurs when chromosomes do not separate correctly during meiosis. The direct result is that one gamete will have an extra chromosome and the other will be lacking a chromosome. When these gametes are fertilized by a normal gamete, they have either an extra chromosome (trisomy) or are missing a chromosome (monosomy).