Each gene runs the risk of changing to an alternative form. The causes of naturally occurring, or spontaneous, mutations are largely unknown. The environment contains a background of inescapable radiation from radioactive elements, cosmic rays and gamma rays. It is conceded that the amount of background radiation is too low to account for a spontaneous mutation can be attributed to background radiation. We may say that spontaneous variations are random and unpredictable.
In 1927, the late Nobel laureate Hermann J. Muller of Indiana University announced that genes are highly susceptible to the action of x-rays. By irradiating fruit flies with x rays, he demonstrated that the process of mutation enormously speed up. T he production of mutation is directly proportional to the total dosage of x rays. Stated another way, the greater the dosage, the greater the mutation rate. In this linear, or straight-line, relationship the mutation rate increases proportionately about three percent for each 1,000 units of x rays.
It has long been held that the mutagenic effect is the same whether the dose is given in short time or spreads over a long periods of time produce as many mutations as the same dose administered in high intensities in a short period of time. Recent experiments on mice have cast some doubt on this view, for it has been shown that the mutagenic effect of a single exposure to the germ cell is greater than the effect of the same exposure administered as several smaller doses separated by intervals of time. Nevertheless, there does not appear to be critical or threshold, dose of roentgens below which there are no effect. In essence, no dose is so low (or “safe”) as to carry no risk of inducing a mutation. Modern workers stress that any amount of radiation, can cause a mutation.
At the time of Muller’s discovery, no one conceive that within a generation the entire population of humans would be exposed to a significant increase of high-energy radiation as a consequence of the creation of the atomic bomb. The additional amount of high-energy radiation already produced by fallout from atomic explosions has undoubtedly increased the mutation rate. Most of the radiation induced mutations are recessive and most of them are deleterious.
In discussing the hazards of x rays and other ionizing radiations, we must be careful to distinguished between somatic damage and genetic damage. Injury to the body cells of the exposed individuals themselves constitutes somatic damage on the other hand, impairment of the genetic apparatus of the sex cell represents genetic damage. Typically, the genetic alterations do not manifest themselves in the immediate persons but present a risk for their descendants in the next or succeeding generations.
There are documented records of the somatic consequences of exposure to the 1945 atomic blasts in Hiroshima and Nagasaki. Among 161 children born of women who were exposed to the atomic bomb while pregnant, 29 were microcephalic (head size considerably below normal) and 11 of these 29 were mentally retarded. As might be expected, the detrimental effects were most pronounced among the infants of women who were in the early stage of pregnancy( less than 15 weeks gestation) at the time of exposure. Moreover, most of the women who gave birth to deformed infants were less than 1.3 miles from the center (hypocenter) of the explosion. The adverse effects on the fetus diminished in frequency and severity as the distance from the hypocenter decreased.
Analyses have also revealed that survivors of the atomic blasts have developed leukemia in rough proportion to the dosage of ionizing radiation. From 0.02 to 1.00 percent of the survivors within the range of radiation developed leukemia in the decade between 1950 and 1960, with the greatest percentage of cases present in individuals nearest the hypocenter. The data substantiate the association found in other studies between whole body exposure to radiation at high dose levels and the incidence of leukemia.
Most mutation present in an individual not only because of mutagenic causes but are also because it has been inherited from the previous generation. The rate at which new disease-causing mutation arise in human is very low. The mutation rate for any single human gene must be considered as a rough estimate. It is likely that all human offspring contain at least one newly mutated gene capable of having an adverse effect. The consensus is that the rate of mutation per gene in the human ranges from one in 100,000 gametes per generation to one in 1,000,000 gametes per generation. Stated another way, any given gene mutates, with a clinically adverse effect, approximately once in every 100,000to 1,000,000 sperm cells or egg cells produced in a generation. A large-sized gene, just as the likelihood of a misspelling in a manuscript is greater in a long sentence than in a short sentence.
By far, most of the gene mutations arising today in organisms are not likely to be beneficial. Existing populations of organisms are products of a long evolutionary past. The genes that are now normal to the members of a population represent the most favorable mutations selectively accumulated over eons of time. The chance that a new mutant gene will be more advantageous than an already established favorable gene in slim. Nonetheless, if if the environment were to change, the mutant gene might prove to be beneficial in the new environmental situation. The microscopic water flea, Daphnia, thrives at a temperature of 20 0C and cannot survive when the temperature rises to27 0C. A mutant strain of this water flea is known that requires temperatures between 25 0C and 30 0C and cannot live at 20 0C. Thus, at high temperatures, the mutant gene is essential to the survival of the water fleas. This little episode reveals an important point: A mutation that is inferior in the environment in which it arose may be superior in another environment. Skeptics might contend that this proverbial declaration has no relevancy to complex higher organism, including human. Interestingly, the maxim derives overwhelming support from the varied frequencies of the gene for sickle-cell anemia in human populations. The obviously harmful sickling gene may actually confer an advantage to its carriers in certain geographical localities.
The sickle-cell anemia was discovered by the American physician James B. Herrick, who in 1940 made an office examination of an anemic African American male residing in Chicago. The patient’s blood examined under the microscope showed the presence of numerous crescent-shaped erythrocytes. the patient was kept under observation for six years, during which time he displayed many of the distressing symptoms were now recognize as typical of the disease
The process of mutation furnishes the genetic variants that are the raw materials of evolution. Ideally, mutation should arise only when advantageous, and only when needed. This, of course, is fanciful thinking. Mutations occur irrespective of their usefulness or uselessness. The mutations responsible for achondroplasia and retinoblastoma ( malignant eye tumors) in humans are certainly not beneficial. But novel heritable characters arise repeatedly as a consequence of mutation. Only one mutation in several thousand might be advantageous, but this one mutation might be important, if not necessary, to the continued success of a population. The harsh price of evolutionary potentialities for a population is the continual occurrence and elimination of mutant genes with detrimental effects thus, in evolutionary terms, a population, if it is to continue to evolve, must depend on the occasional errors that occur in the copying process of its genetic material.
In related to the evolutionary process, human as we can see are having only a single species, Homo sapiens. One of the factor is that the mutation that occur in human ore not cause a big alteration in humans phenotypic. Different population of humans can interbreed successfully and, in fact, do. The extensive commingling of populations renders it difficult, if not impossible, to establish discrete racial categories in humans. Races are geographically defined aggregates of local populations the population of humankind are no longer sharply separated geographically from one another. Multiple migrations of peoples and innumerable intermarriages have tended to blur the genetic contrasts between populations. The boundaries of human races, if they can be delimited at all, are the best fuzzy, ever shifting with time.
The nature and circumstances or human life have been profoundly altered in the last few decades. Cultural innovations have occurred on an unprecedented scale. Cultural changes provide potential opportunities and potential disasters. Each innovative cultural change is rooted in new knowledge and is accompanied by new outlooks and responsibilities. However, too open new information is distorted to fit preexisting patterns of attitudes belief, and actions. Despite the onrush of new knowledge, humans still remain encumbered by dogmas that trace their roots to early historic times. People are generally more comfortable in adhering to old value. However, many of our old values and prescribed actions are much too inflexible to cope adequately with the profound changes and variety of real-life situations that we are witnessing today. Adjustment in values have to be made when new knowledge in a changing situation does not reinforce past values based on a different set of circumstances.
References:
E. Peter Volpe & Peter A. Rosenbaum : Understanding Evolution, 6th edition; Mac Graw Hill publication
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