History Of Genetics Essay Research Paper What

History Of Genetics Essay, Research Paper

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 (Ap