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Medical Genetics

patient disease molecular diseases

Medical genetics is a multi-dimensional study of medically significant human genetic variation.

Molecular diagnostics is the newest specialty field in genetics and allows a better understanding of heredity and disease at the molecular level. Disorders due to mutations as small as a single base change in the DNA can now be explained. Once the mutation is known, clinical laboratory testing can usually be set up. For diseases such as cystic fibrosis, Huntington disease, and fragile X syndrome, molecular assays are now the standard of care. The next step is to look for ways to reverse or repair the defect or to completely replace the mutation with a fully functional gene. Advances in these areas provide the foundations for gene therapy.

A wide range of disciplines, including—but not limited to—molecular, cellular, organismal, and population biologies, contribute to advances in medical genetics. Although once thought to be a relatively esoteric science, genetics now plays a key role in essentially all subspecialties of medicine. It has been suggested that, with the exception of trauma, genetics may be the underlying cause of all medical complaints. herefore, when a gene that causes a particular disease is identified, it becomes important to understand the inheritance and expression of that gene.

The era of medical genetics started when Archibold Garrod identified the first genetic diseases, the inborn errors of metabolism. Over the years, other similar disorders have been identified and were shown to be caused by defects, or mutations, in particular biochemical pathways. Examples include phenylketonuria (PKU), Tay Sachs disease, hypercholesterolemia, cystic fibrosis, and galactosemia. These are often progressive diseases with severe mental and physical complications that may result in the death of the patient. In some cases, the negative consequences may be avoided if the disease is treated early. Because of this, most states now mandate a newborn screening program to evaluate all babies for several of the most severe disorders. Another recent discovery is that inherited genes play a role in drug metabolism and the efficacy of drug treatment. The new field of pharmacogenetics is beginning to elucidate to complex role of genes as they relate to this aspect of biochemical genetics.

Cytogenetics is the oldest of the medical genetic sciences, but it did not become important clinically until 1959, when the presence of an extra copy of chromosome 21 (Trisomy 21) was first implicated in Down syndrome. Since then, many other chromosomal abnormalities have been directly associated with different diseases. By viewing metaphase cells with the light microscope, trained personnel can detect a broad range of numerical and structural chromosome anomalies that can then be correlated to specific diagnoses ranging from Down syndrome and Turner syndrome to various types of cancer. Recent collaboration with molecular geneticists has provided a new tool, fluorescence in situ hybridization, or FISH, that allows visualization of smaller abnormalities using fluorescently labeled molecular probes.

For the patient, the key to medical genetics is the physician, or clinical geneticist, who evaluates the problem and makes the diagnosis. The medical history, pedigree, and physical examination of the patient are reviewed to narrow the possibilities, and then laboratory testing is performed. The final diagnosis is based on all of the information obtained. The clinical interpretation, potential treatment, prognosis, and recurrence risks are then provided to the patient and the patient's family with the aid of a genetic counselor. Genetic counselors are very important in this process since they are specially trained to turn complex medical jargon into terms patients can easily understand. Once these steps are completed, much of the care and treatment of the patient can be carried out by other medical specialists.

Medical genetics therefore reflects a team approach to patient care with input from each of the relevant specialty areas (clinical genetics, cytogenetics, molecular genetics, biochemical genetics, and genetic counseling) as well as clinicians with expertise in other areas of medicine. Genetics is, however, a continually evolving science, so in addition to clinical endeavors, medical geneticists are active in research. Spearheaded by the Human Genome Project, genes that cause disease are continually being identified. In time, better laboratory tests, more effective treatments, and potentially cures for serious diseases will be available. This represents a major advance over the time when medicine was content to describe a disease and utilize treatments that were merely palliative (i.e., although a patient felt better, he or she still had the underlying disease). Medical genetics represents the promise of medicine for the future—the ability to provide targeted cures for diseases.



Strachan, T. and A. Read. Human Molecular Genetics. New York: Bios Scientific Publishers, 1998.


Hyman, S.E. "The Genetics of Mental Illness: Implications for Practice." Bulletin of the World Health Organization 78 (April 2000): 455–463.

Shields, R., "The Emperor's New Clothes." Trends in Genetics 17 (2001) 189.

Constance Stein

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