Chromosomal Aberrations

A deletion is the loss of a segment of a chromosome. The amount of deleted material may be any length from a single base to a large piece of the chromosome. The result is a chromosomal imbalance, with the individual being monosomic or possessing half of the required genes present in a normal individual for the segment of DNA missing. Only small deletions are tolerated, and the effect on the individual will depend upon the size of the deleted segment and the number and functionality of the genes that are contained within it. Larger deletions and the deletion of an entire chromosome always result in nonviable embryos. Cri du Chat's ("cat's cry") syndrome individuals have a deletion of the short arm of chromosome 5. Although they possess the usual signs of chromosomal anomalies, such as mental retardation and low birth weight, their appearance is not extraordinarily different from normal individuals. One peculiarity is that affected infants make an unusual cry resembling that of a cat, hence the name of the syndrome. Two other interesting diseases are Prader-Willi's syndrome and Angelman's syndrome. In both cases, patients with these diseases possess a deletion in the long arm of chromosome 15. Interestingly, the deletion is in the same location, but the resulting syndrome depends on whether the deletion was in the maternal or paternal chromosome.