Altering the stuff of life
By Robert A. Hedeen, Naturalist
Charles Darwin published his book, The Origin of Species by Means of Natural Selection, in 1859, and about that time Gregor Mendel, an obscure Austrian monk, initiated work in a small garden at his monastery that would lay the foundation for the science of genetics. Mendel answered questions for which Darwin did not have the answers. Mendels experiments showed how certain characteristics may be either dominant or recessive, how traits could skip a generation, and how certain characteristics in the offspring could appear when neither of the parents possessed them. Unfortunately, Mendels work was published in an obscure journal and not noted until about 1900 when it was discovered.
One of the men who rediscovered Mendel was the Dutch botanist Hugo De Vries, and he conducted experiments to explain exceptions to Mendels basic laws of inheritance. What we call genes today, Mendel called determiners, and de Vriess studies revealed that determiners sometimes suddenly and without warning underwent drastic changes, and these variations from the norm were permanent and passed on to the offspring. Today, we call these changes in the genetic material of living things mutations.
For more than a quarter of a century, scientists were unable to determine what caused mutations to occur. A breakthrough came in 1927 when Dr. Herman J. Muller, at the University of Texas, was able to induce mutations in fruit flies by bombarding them with X-rays (Muller was later awarded the Nobel Prize in medicine for founding the science of radiation genetics).
Mutations have been studied extensively for the past 75 years, and a great deal has been learned about them. In general, there are two basic types of mutations that occur in the genetic material of living organisms: point or genic mutations that occur in a specific gene, and chromosomal mutations that affect the chromosomes where genes are located. The slightest change in the sequence of the components of a DNA molecule can result in a drastic way the manner in which the resulting gene expresses itself. Likewise, as every species of animal or plant has its own unique number and arrangement of chromosomes, any alteration of this basic plan profoundly influences the organism. Anything that causes either a genic or chromosomal mutation is called a mutagen, and more and more of these agents are being constantly recognized.
Radiation of various types is a very important source of both genic and chromosomal mutations. X-tays are not, however, nearly as potent a mutagen as other types of radiation, e.g., gamma rays, but one should avoid overexposure to them. That is why the dentist always covers your chest with a protective shield and retreats behind a barrier when an X-ray is made of your dentition.
In addition to radiation, certain chemicals are known to be mutagens. In the early 1940s, Drs. Auerbach and Robson, working at the University of Edinburgh, found that mustard gas produced chromosomal mutations, and the first chemical mutagen was discovered.
In 1962, Rachel Carson, in her award winning book Silent Spring, called attention to the mutagenic effects of the commonly used chlorinated hydrocarbon type insecticides such as DDT, chlordane, aldin, dieldrin, and benzene hexachloride. These very efficient bug killers were long lasting and relatively inexpensive to produce, but in addition to having mutagenic effects, were found to be detrimental to non-target animals in many other ways. These insecticides were finally banned for general use in the early 1970s. Carson also pointed out the mutagenic effects of certain widely-used herbicides, such as the infamous Agent Orange, used extensively during the Vietnam War.
Herman J. Muller, based on decades of research with mutagens after his initial X-ray work, warned that various chemicals can raise the mutation rate as much as radiation: Far too little is known of the extent to which our genes and chromosomes, under modern conditions of exposure to unusual chemicals, are being subjected to mutagenic influences. We should be extremely suspicious of the broadcasting of pesticides over wide areas and be sure that, in addition to toxicity tests, mutagenic potentials have also been performed by the appropriate agency. Likewise, every step should be taken to ensure that mutagenic chemicals do not find their way into our food and drinking water.
Most mutations are deleterious to living organisms, but some are not. Those that prove to be beneficial to an organism supply the raw material of evolution. Some mutations are thought to be without cause or to be spontaneous in nature. The rate of spontaneous mutation has been accurately estimated in humans, and it is quite probable that each of us has, for better or worse, at least one gene that neither of our parents possessed.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.