cient bacteria that migrated into animal cells and became essential functional parts of those primitive cells. Sperm lose their mitochondria when they penetrate the egg; thus, mitochondrial genes (and any mutations in them) are passed only from the mother. Mutations in mitochondrial genes can be detected by molecular techniques and have been positively associated with certain types of blindness (Leber optic atrophy), muscle diseases, a type of epilepsy, and dementias associated with aging.

Another type of inheritance is known as allelic expansion. Here, instead of DNA remaining constant over the generations, as is usually the case, a gene segment expands in size when transmitted from parent to child. The expanded gene may cause a characteristic disease such as Huntington disease, myotonic dystrophy, spinal bulbar muscular atrophy, or fragile X mental retardation syndrome. Disease severity may be related to the size of the expanded gene. Allelic expansion accounts for a phenomenon called anticipation, in which there is earlier onset and/or increasing severity of disease as the expanding gene is transmitted from generation to generation. Molecular tests for allelic expansion are already available.

Mosaicism is the existence of cells with different genetic constitution in the same organism and is of greatest clinical importance in cancer. Cancer cells usually carry genetic mutations not shared by the normal cells. The organism affected with such a mutation is a somatic mosaic, where the cancerous tissue often has a different genetic constitution from the rest of the body. Germinal mosaicism affecting gonadal cells (sperm- and egg-forming tissue) also occurs. The finding of several affected siblings with an unexpectedly nonaffected parent in diseases that are transmitted by autosomal dominant inheritance may be due to germinal mosaicism. In such cases a section of the parent's gonad carries the diseaseproducing mutation.

Technologies for Detecting Genetic Disorders

Technologies to detect genetic disorders have existed for some time but have expanded dramatically in their scope, accuracy, and speed over the past 20 years. The earliest forms of genetic diagnosis, still frequently practiced, were based on observation of an individual's clinical findings or constellation of anomalies, and on an assessment of the family history. Later, biochemical assays were developed to test for inborn errors of metabolism, such as phenylketonuria or sickle cell disease. These earlier techniques remain useful today. Chromosomal analysis has been in use for more than 30 years to diagnose errors in number or shape of chromosomes that can result in genetic disorders and disease. Chromosomal analysis is most often practiced in the evaluation of newborns with malformations and for prenatal diagnosis for advanced maternal age. (As a female ages, the eggs she carries are more likely to produce errors in meiosis, leading to an increased risk of bearing a child with a chromosomal anomaly.) Advances in DNA technology

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