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PERSPECTIVES IN MEDICAL GENETICS N. H. HOROWITZ* Introduction This year marks the iooth anniversary of Mendel's famous paper on inheritance in peas. It is particularly fitting, therefore, that the organizers ofthis meeting have decided to hold a colloquium dealing with genetic problems in medicine. To me has fallen the assignment of discussing perspectives in this field. To obtain a perspective, one can look either forward or backward. In this centennial year, many speakers will look backward over the long road that genetics has traveled since Mendel delivered his report to the Natural History Society ofBrunn. The view is an inspiring one. The units that we now call "genes" were, in Mendel's paper, mere mathematical abstractions—entities that he invented in order to account for the numerical results of his breeding experiments. Today, the genes are real chemical substances, located in special organelles ofthe cell. The chemical structure ofgenes and their mode ofreplication are, for the most part, known, and their mode of action is understood. These advances in our knowledge of the genes have revolutionized biology. The creation ofmolecular genetics, starting from the laws ofMendel, is surely among the greatest accomplishments oftwentieth-century science. The spectacular progress of modern genetics has convinced some thoughtful biologists that we are on the threshold ofa new era in which man will be able to control—and, indeed, to remake—his own heredity [i]. Control ofhis heredity would give man the capability ofdirecting his own development and evolution. The prospects are, to say the least, * Professor ofBiology, California Institute ofTechnology. This paper was presented at the Third Panamerican Symposium on Pharmacology and Therapeutics held in San Luis Potosí, Mexico, August 4-7, 1965. It will be published in Spanish by Excerpta medica (Spanish edition). Results from the author's laboratory were obtained under Grant GB-444 from the National Science Foundation. 349 staggering. Even the remote possibility ofsuch power compels us to pause and consider the situation. In this paper, I should like to examine the medical genetic aspects ofthis question. The Scope ofMedical Genetics Medical genetics in its most narrow sense refers to genetic defects that are transmitted from parent to offspring. These defects are inherited in a Mendelian way, and they are usually congenital. Typical examples are galactosemia, phenylketonuria, sickle-cell anemia, and hemophilia. Also in this category ofcongenital afflictions are those, such as mongolism, which result from chromosomal aberrations of one kind or another. Congenital defects such as these form only a small part of the genetic diseases ofman, however. Much the larger part fall into a category that, for convenience, I shall refer to as somatogenetic diseases. These are defects that result from somatic mutations acquired during life. It is admittedly difficult, ifnot impossible, to prove that any disease is caused by somatic mutation, since, by the nature ofthe case, we are precluded from applying the breeding tests by which gene mutations are operationally defined. Nevertheless, there are strong reasons for thinking that somatic mutation is an important cause ofdeath in man. It has long been suspected that cancer is caused by somatic mutations. Indeed, this must be true almost by definition, since cancer cells are permanently altered cells that transmit their abnormalities to their daughters through an indefinite number of mitotic generations. The real problem ofcancer etiology is to delineate the chain ofevents that terminates in the appearance ofthe mutated cell [2]. It is, of course, well known that cancer can be produced by ultraviolet light andX-rays, which are powerful mutagens. The fact that the incidence ofleukemia in man increases with the dose ofionizing radiation in an apparently linear fashion [3, 4] suggests that, as in the case ofgene mutations, leukemia can be caused by a singlehit in the chromosomes. It is known that some cancers have a viral origin. Viral diseases, too, come under the general definition ofsomatogenetic afflictions, since viruses produce their effects by insinuating their own genomes into the genetic machinery ofthe cell. Also of great interest in connection with somatogenetic effects is the evidence that ionizing radiation accelerates the aging process in animals 350 N. H. Horowitz · Medical Genetics Perspectives in Biology and Medicine · Spring 1966 [5, 6]. Experiments in a number...


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