History Of Genetics

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What is Genetics? Genetics is usually defined as the transmission of traits from one generation to the next. Although correct in its meaning, the definition is rather vague. Genetics not only involves the transmission of traits from generation to generation, but it also involves every biological occurrence in an organism. The history of genetics, beginning with the ideas of Aristotle up till the rediscovery of Mendel's work, has gone through many changes both in theory and discovery. The history of Genetics most often begins with the ideas of Aristotle and Hippocrates. Their basic belief on Genetics included the determination of the sex and inheritance of disease based on the idea of Spontaneous Generation. They believed that sex of the offspring depended on which part produced the sperm that fertilised the egg. Through this, Darwin (Appendix 1, picture 4) later called the theory "Pangenesis". He believed that gemmules (1) were manufactured by every part of our body, which then collected in the semen producing the basis of heredity. "Although Pangenesis was believed by most people, Aristotle came to the conclusion that characteristics were not inherited, but the ability of producing these characteristics were" (Sturtevant, 1965, p2). Another theory that was proposed during this time was Preformation. It stated that whole miniature individuals lived in the germ cells and matured in the womb of the female. It was unknown during this time how traits were passed, so scientists concluded that somehow aspects of the parent's bodies were transferred in miniature individuals known as homunculus (2). As we entered the 18th and 19th century, the improvement of the microscope helped to disprove the Theories of Spontaneous Generation and Preformation. With this, the question of how traits were inherited was still unknown. Gregor Mendel (Appendix 1, picture 1), better known as the Father of Genetics, was the first scientist to show that traits had a predictable pattern. He had succeeded were many others failed by luckily choosing simple and unchanging traits. He established; according to the thousands of crosses he made; that there was a pattern of transmission of traits. "Resulting from his studies was the Law of Segregation and the Law of Independent Assortment" (Sutton, 1988, p11). After completion of his eight years of investigation, Mendel presented his work upon the Science Research Society. The significance of his work was not realised until 1900 when his work was rediscovered. Three years after Mendel had completed his work; a German scientist named Friedrich Miescher discovered nuclein (3). He believed that this substance was storage for phosphorus (4) rather than genetic material. During this time, it was believed that protein was the basis of heredity because it was so complex. They didn't believe that DNA was the hereditary factor because it was so simple and easily understood. Miescher's discovery did not realise as an importance until 1889, with the development of August Weisman's Germ Plasm (5) Theory. It suggested that "each chromosome remained intact from generation to generation and it was passed by the germ cell. He also concluded that each chromosome contains all hereditary elements to produce an individual" (Sturtevant, 1965, p19). In other words, the chromosomes were responsible for the transportation of hereditary material. The Germ Plasm Theory gave rise to the Chromosome Theory of E.B. Wilson. It stated that chromatin is very similar to nuclein and that inheritance might be effected by the transmission of chemical compounds from parent to offspring. Both the Germ Plasm Theory and Chromosome Theory explained nuclein as DNA and DNA as genes (Appendix 1, picture 2). "Walter S. Sutton developed his own theory and it suggested that chromosomal pairs were equally important as the segregating pair of gene alleles (6)" (Sturtevant, 1965, p31). Correns and Hugo De Vries assisted Sutton's Chromosome Theory. Correns assumed there were different orders of alleles that allowed recombination. Sutton then ran into a problem with his theory when he noticed there were not enough chromosomes to identify each gene in a whole chromosome. "Hugo de Vries proposed that sometimes those genes were possibly exchanged freely during meiosis (7). With this developed the Theory of Crossing Over" (Sturtevant, 1965, p39). The phenomenon of crossing over is the exchange of genetic material between two or four chromatids of a tetrad during synapsis (8). "These chromatids join at a point called the chiasmata, and it is there were segments of chromatids are exchanged" (Pai, 1974, p98). It plays a big role in the rearrangement of alleles into different recombinations, which leads to genetic diversity. Otherwise, chromosomes would remain the same except for and occasional mutation. "After the crossing over process, one can no longer distinguish between maternal and paternal chromosomes since the DNA is now combined" (Sturtevant, 1965, p39). The discovery of linkage eventually resolved this difficulty. Bateson, on the other hand, was the first to report incomplete linkage. "The result of this made the estimation of the recombination of genes very difficult and imperfect" (Sturtevant, 1965, p40). With the discovery of linkage, de Vries Theory of Crossing Over (Appendix 1, picture 3) was proven to be correct. The first suggestion of a particular characteristic to a particular chromosome was made in 1901. C.E. McClung believed the X chromosome was the male determining factor. He came to this conclusion by counting 22 X chromosomes in the female and 23 X chromosomes in the male. Since the male retained one more X chromosome than the female, he believed it to be the main factor in the determination of sex. McClung's assumption revealed to be incorrect. In 1905, N.M. Stevens showed the correct relationship to be as followed: "the presence of XX chromosomes showed clearly to be a female and the combination of the XY chromosomes resulted in a male" (Sturtevant, 1965, p41). Through further studies, the most important element was shown to be that Y relating sperm produced male offspring and X bearing sperm produced female offspring. Another important factor in the study of genetics is continuous variation. Galton discussed this in his Law of Ancestral Inheritance. It correlated the resemblance between parent and offspring. As discussed in H.J. Muller's Variation Due to Change in Individual Gene," there were thousands of genes that played an important role in determining cell substance and cell structure" (Muller, 1922, p105). Galton's Law of Inheritance is an alternate interpretation due to Mendel's principle of random breeding. The introduction of environmentally produced variation developed another method of studying the inheritance of characteristics by selection. Most of the effects of selection are due to the sorting out and build up of modifying genes. "These genes then produced characteristics of an organism to help them become more adapted" (Muller, 1922, p105). This adaptation implies the expressed variation due to changes in individual genes or better known as mutation. By this, it was determined that selection works on genes already present in the organism. "The understanding of how selection operates has become very important in applying genetics to the problems of evolution" (Sturtevant, 1965, p61). Muller introduced another method by which he believed genes affected characteristics. It suggested there was some connection between chromosome behaviour and gene structure, but he insured us that it was only a possibility. The development of genetics is largely due to Gregor Mendel. He discovered what took many scientists many years to uncover. Mendel's papers first appeared in 1866 and were ignored because they were not understood. In 1900, when the time was right, other geneticists discovered the principles. Believing they had discovered something nobody knew; some scientists did not want to accept the fact that Mendel had made the discovery years before them. Through the years genetics has explained topics such as diversity, inheritance and sex determination. From this, there is no doubt that genetics is the basis of evolution.

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