Progress in genetics since Mendel.
The Mendelian principles stimulated a great many biologists to undertake definitely planned breeding experiments during the first few years following 1900. It soon became apparent that some crosses seemed to uphold Mendel's work, whereas others appeared to contradict it. Confusion soon set in and geneticists realized that Mendel's simple explanations were inadequate to account for all of heredity. In spite of this, it became apparent that Mendel's basic principles were sound, but limited, and that he had not taken into consideration all phases of heredity. The years following 1900 have added many interesting new conceptions to the science of genetics and the most important of these may be summarized briefly along the following four main lines.
First may be mentioned the elaboration of the multiple factor hypothesis. Many traits depend for their expression on multiple factors, that is, more than just a pair of them. In some instances, it has been shown that as many as a dozen factors may be necessary for the expression of a trait. Where one or more of these factors is missing, a modification takes place. Degrees in the expression of a trait may be accounted for largely on the operation of multiple factors, particularly so-called modifying and cumulative ones. In line with the elaboration of the multiple factor hypothesis has been the establishment of the fact that the terms dominant and recessive are relative. Some factors are either completely or partially dominant, while others are neither dominant nor recessive; but instead, they cooperate to bring about different results.
A second line of advance has been the establishment of the gene theory of heredity. This hypothesis asserts that definite entities within the chromosomes are responsible for the expression of traits. These entities have been called genes and through their cooperation the characters of organisms are believed to be established. The elaboration of this gene principle was due largely to the work of Prof. Thomas Hunt Morgan and his students on the fruit fly, Drosophila melanogaster. It was fortunate for genetics that in 1907 Professor Morgan found that this insect is a suitable animal for breeding experiments. Not only does the fruit fly breed very quickly but also it is easy to confine and handle within the biological laboratory. Moreover, crosses between different types of these flies can be made very readily and results can be obtained very quickly. Professor Morgan surrounded himself with a group of alert workers who attacked Drosophila from various standpoints, particularly cross-breeding and cytology. As a result of these investigations facts and principles were revealed that led to the formulation of the gene theory of heredity. Following this the question soon presented itself whether the genes may not be located and associated within the distinctive chromosomes. Soon this was answered in the affirmative and since then geneticists have been engaged in plotting the location of the factors for the traits of the fruit fly within the respective chromosomes. At the present time students of genetics are familiar with the four linkage groups of Drosophila embracing over 500 genes and representing distinctive groups of traits in this interesting insect.
A third line of advance since Mendel has been the utilization of X-rays and radium emanations for purposes of inducing permanent modifications in organisms. Such changes are called mutations and one of Professor Morgan's former students, Dr. Herman J. Muller, has been the pioneer in this line of research. By exposing vinegar flies to strong emanations of X-rays, Muller caused the progeny of these organisms to vary approximately 15,000 percent, more than under normal conditions. Moreover, these modifications have proven to be just as permanent and heritable as the mutations which arose in nature. Changes were observed in nearly all parts of the animal, in the wings, eyes, legs, and other bodily structures. This was a monumental discovery and soon other investigators, notably Stadler, Hanson, Goodspeed, Mavor, and Patterson, by the use of similar emanations succeeded in inducing comparable permanent modifications in various other animals and plants. It thus becomes evident that the experiments with X-rays and similar emanations have afforded the biologist a means of bringing about certain degrees of evolution within the confines of his laboratory.
Careful examinations of the germinal contents of the organisms modified by the above means have led to the conviction that the changes undoubtedly have been produced in one of the following ways:
Through distinct transformations within the genes themselves.
Through the fragmentation of one or more of the chromosomes representing linkage groups and the translocation of such fragments to other chromosomes. When a germ cell showing such a translocation participates in fertilization, the additional genes on one or more of the linkage groups may be responsible for the production of permanent modifications.
Through destruction or deletion of portions of chromosomes, certain genes may be eliminated, thus making it impossible for these destroyed factors to gain expression.
Finally, through non-disjunction some of the paired chromosomes representing paired linkage groups may not separate during synapsis and therefore more than the ordinary simplex number may be present in a portion of the mature germ cells. When these participate in reproduction they may be instrumental in initiating permanent modifications leading to the emergence of new types of organisms.
The most recent line of advance since Mendel came in 1932 when it was shown by Dr. Painter of the University of Texas that the salivary gland chromosomes of Drosophila, which are veritable giants, being over a hundred times as large as the chromosomes of the reproductive cells, afford splendid opportunities for studying the structure of the chromosomes, particularly the arrangement and configuration of the lines or bands that run across them. These bands seem to have a peculiarly distinctive pattern in each chromosome and the workers in this field of research are confident that the bands are the regions in or near which the individual genes are located. Since the chromosomes of the salivary gland cells are so much larger than those of the germ cells, it becomes possible not only to recognize modifications that have taken place in the various linkage groups, but also, to map more accurately the locations of the genes within them. There are some biologists who seriously question the latter conclusion, for they point out that in spite of all that has been said and done regarding the genes no one has as yet been able to isolate or reveal any of them. Be that as it may, most modern geneticists are confident that genes actually are in existence and that through their cooperation the foundations for the characters of organisms are established.
By Nathan Fasten
ORIGINALLY PUBLISHED IN MAy 1940
Science long forms a backbone of the magazine. One example: Nathan Fasten (1887-1953), who specialized in zoology and physiology, worked for decades in academia, largely at Oregon State College as a professor and department head, and later was chief scientist for the Washington State Water Pollution Commission in Seattle. He authored many articles and books.