CHROMOSOME STRUCTURAL ABNORMALITIES
Human disease also results from abnormal chromosomal structure due to translocations, deletions, and fragile sites. In one type of translocation, part of a chromosome breaks off and reattaches to a different chromosome (which results in extra copies of the translocated genes). In about 4% of Down syndrome sufferers, an extra piece of chromosome 21 is translocated onto a larger chromosome, such as chromosome 14.

Sometimes chromosomes break but fail to rejoin. Such breaks result in deletions of base pairs. Large deletions are lethal, and small deletions cause several recognizable disorders.

Fragile sites occur on various autosomal and sex chromosomes. One, on the tip of the X chromosome, results in fragile X syndrome. This causes mild retardation in about 80% of boys and 35% of girls.