DNA-based tests have also found great value in detecting the potential for genetic disease. Frequently a genetic disease cannot be diagnosed until its symptoms appear. With DNA tests, however, it is possible to locate the genetic basis for the disease. The individual can then be provided with counseling on what the future holds and advised on how to limit transmission of the gene. It may also be possible to institute gene therapy.



Victor McKusick of Johns Hopkins University has assumed the responsibility of cataloging all known genetic diseases in his voluminous book Mendelian Inheritance in Man. In its first edition (1966), his book listed 1500 phenotypes of diseases then known to be inherited. In its current edition, the book has grown to 9000 phenotypes, most of which are linked to certain genes. Often regarded as a founder of the science of medical genetics, McKusick was honored in 1997 by the Lasker Foundation for a lifetime of achievement in science. At this writing, DNA probes are being used to assist the diagnosis of such genetic diseases as sickle cell anemia, hemophilia A, Tay-Sachs disease, and phenylketonuria (PKU).

Every so often in scientific history a window opens and, suddenly, the theoretical becomes possible, then inevitable. Discoveries emerge in rapid succession and powerful new technologies drive researchers to unimagined heights.

One such window opened in biology with the successful use of DNA matching technologies. In these procedures, synthetic pieces of DNA called gene probes stick like magnets to their chromosomal mirror images. Then a biochemical signal indicates that a match has taken place. The techniques permit a broad series of ultrasensitive genetic analyses, including the identification of numerous genetic diseases.

Another result of DNA matching techniques has been the process of DNA fingerprinting, which is used to link a suspect's DNA to the DNA recovered at a crime scene. Already used in thousands of criminal investigations, DNA fingerprinting helped exonerate or convict numerous suspects.

The same DNA technology is helping to decide court cases where there is a question of paternity. Each year, more than 100,000 cases of disputed parentage are heard in courts in the United States. Before DNA matching techniques became available, the court's decision was based largely on the often imprecise determination of blood groups. Genetic analysis can match father to child with a much higher degree of accuracy, a degree estimated to approach ninety-nine percent.

Also taking advantage of DNA analysis techniques are the forensic pathologist and the biological sleuth. Forensic pathologists have discovered that DNA from the skeleton of a long-deceased human can be extracted and used to ascertain that individual's identity. Biological sleuthing has become the new wave of study in anthropology and evolution, as molecular biologists seek to obtain and clone DNA from fossils (including from humans) that are millions of years old. Questions such as the possibility of Marfan syndrome in Abraham Lincoln and the origins of the first Americans may be answered through DNA matching techniques. Each of these topics will occupy our attention in this chapter.