Individual Organism's Hereditary Material example essay topic

What is Heredity? Research Essay on Heredity heredity {hur-ed'-i-tee} Heredity is the transmission from one generation to the next of factors that determine the traits of offspring. Although successful breeding of plants and animals was practiced by humans long before modern civilizations were established, there is no evidence that these early people understood the nature of hereditary factors or how they are transmitted through reproduction. EARLY HISTORY One of the early Greek philosophers, Pythagoras (582-509 BC), postulated that all traits of an offspring are derived solely from its father's semen. Aristotle thought that females also produce semen and that the embryo is formed by a fusion in the uterus of both types of semen.

He further postulated that both male and female semen are produced by the body's blood. Leeuwenhoek Until the 17th century, European medical schools taught that hereditary factors in the semen were derived from vapors emanating from each body organ. However, Anton van LEEUWENHOEK observed human semen through his microscope and reported finding "animalcules". It became generally accepted that sperm were the actual carriers of hereditary factors from males to their offspring. Other biologists studied the ovaries of animals, noted the presence of swollen bodies -- which they correctly assumed contained eggs -- and hypothesized that these eggs were also units of transmission of hereditary factors. Epigenesis Some biologists of the 17th and 18th centuries believed that they saw miniature individuals in the sperm or eggs of various organisms, an observation that led to the doctrine of preformation.

According to this theory all parts of the adult are already formed at the beginning of embryonic life, and as a result, embryonic development consists solely of growth. Toward the end of the 18th century, Caspar Friedrich WOLFF conducted extensive investigations on developing chicken embryos. He demonstrated that the adult parts of the animal are not present at the beginning of embryonic life but are formed during the developmental period. His doctrine of DEVELOPMENT, known as epigenesis, has been substantiated by countless observations and experiments. It is important to note that the biologists who disproved preformation and advanced the idea of epigenesis 200 years ago still held beliefs similar to those of the ancient Greeks on the origin of the hereditary material.

The 18th-century scientists thought that the individual body organs produced tiny particles that had the potential of forming in offspring the same structures as that of the parent. These biologists postulated that the particles from the various organs would be transferred to either sperm or egg, which, upon fusing, would have the potential of forming a total individual. LAMARCK Jean Baptiste LAMARCK strongly promoted the concept of the inheritance of acquired characteristics, which supposes that a parent's organs act individually to produce the hereditary factors that form corresponding parts in the offspring. Any changes that occurred in an organ before a person transmitted genetic material to offspring would result in the production of an hereditary factor that would reflect the altered organ.

The kinds of changes that could be inherited would be those that result from either increased use or disuse of an organ, or those changes resulting from environmental factors, such as disease or accident. The implications of this theory were important not only for the inheritance of traits from one generation to the next, but also for the long-term evolutionary changes of a species. Lamarck believed, for example, that the children of weightlifters would have strong arm and shoulder muscles. He also stated that giraffes have long necks because their ancestors continually stretched their necks to reach for and eat the leaves on tree branches.

DARWIN Perhaps the most significant event in the history of biological research and thinking took place in the 19th century, when Charles DARWIN published his works on the theory of EVOLUTION. According to Darwin's theory, traits vary considerably among the members of a particular population. In competition for the limited resources, there results a "survival of the fittest" -- certain plants or animals are better able to survive and reproduce than others; "fittest individuals" owe their success to having hereditary factors different from those of the rest of the population. Their offspring will resemble the parents, and, after many generations of this "natural selection" process, the characteristics of the population will be quite different from those of the ancestral group.

Because the information was not known at that time, Darwin was unable to explain correctly how variation in traits occurred. He accepted the Lamarckian view, which emphasized the role of environment as well as the use and disuse of organs. In 1901, Hugo de VRIES proposed his mutation theory, which states that the variation seen in many traits among members of a given population is the result of changes that have occurred in the individual organism's hereditary material. This view has been well substantiated, and it remains the explanation for the origin of altered hereditary characteristics. MENDEL'S LAWS A greater understanding of the transmission of hereditary factors from one generation to the next was achieved (1900) with the rediscovery of the findings of Gregor Mendel, an Austrian monk who performed his experiments on garden pea plants and developed (1865) his theory of inheritance. Mendel's laws have remained an important basis for the study of heredity and genetics.

In his experiments Mendel cultivated individual garden peas that came from purebred lines and had always shown a particular trait. He crossed these parental plants with contrasting traits, such as yellow versus green seed color, and studied the offspring, which are called the first filial generation. Mendel found that all the first generation individuals showed one particular trait, namely, the dominant trait; later generations that were crosses of first generation hybrids resulted in certain ratios of occurrence of dominant and recessive traits. From his observations Mendel hypothesized the law of dominance.

It has since been learned that there are many hereditary traits in which there is no dominance between contrasting characteristics, but rather a blending effect between them, as in skin color in human beings. Mendel also postulated the law of segregation, which states that only one of each pair of hereditary units enters a particular gamete, and the law of independent assortment, which states that each dominant or recessive trait is inherited independently of every other one. 20 TH-CENTURY RESEARCH The association of chromosomes with hereditary units was not established until 1902, when Walter S. SUTTON observed that chromosomes were present in pairs in the regular cells of a grasshopper. In the formation of reproductive cells, only one member of each pair of chromosomes entered a sperm or egg. During fertilization, the subsequent fusion of sperm and egg restored the normal cell chromosome number. Sutton quickly saw that the pairing of chromosomes, their separation in the formation of sperm and egg, and their subsequent rejoining during fertilization paralleled the movement of the "hereditary units" in Mendel's experiments.

The establishment of the chromosome as the carrier of the genes led to a great deal of research to identify the hereditary material. This culminated in the discovery in 1944 by Oswald T. AVERY, Mac lyn MCCARTY, and Colin M. MACLEOD that, except in some viruses, DNA was the hereditary material in all organisms. Polygenes Biologists began to study the inheritance of such complex traits as height, weight, and skin color, none of which show simple Mendelian dominance. Sir Francis GALTON did the most to establish this line of research, which is known as quantitative inheritance. The action of many genes, or polygenes, is required to produce this type of body characteristic. Although the individual hereditary units follow the laws of segregation and independent assortment, as do those that Mendel studied, the individual genes cannot be independently studied.

Unlike Mendelian genes, in which a single gene dictates whether a pea seed will be yellow or green, no discrete classes of polygenes exist. Polygenes, however, do have a continuous range of values, for example low-to-high or less-to-more. Cytoplasmic Chromosomes Although most traits of the body are determined by genes in chromosomes located in the cell's nucleus, chromosomes also exist in the cytoplasm of the cell. The presence of hereditary units in the cytoplasm was first hypothesized in 1902 by Karl Corres (1864-1933), who found that chlorophyll-containing cell bodies of plants (chloroplasts) are transmitted from one generation to the next by means of the egg, not the sperm.

Subsequently, it was found that certain traits associated with the energy-producing cell bodies (mitochondria) are also transmitted only through the female. This results from the fact that eggs contain a great deal cytoplasm and numerous cell bodies. Sperm, however, are virtually devoid of cytoplasm and usually carry no cell bodies. Chloroplasts, mitochondria, and other cell bodies (flagella and centrioles) contain DNA that determines some of their traits. This uni parental type of inheritance is called non-Mendelian inheritance. It has taken on increasing importance since cytoplasmic chromosomes, called PLASMIDS, were discovered in bacteria.

The plasmids control such traits as resistance to antibiotics. It has also been found that genetic material from other species of organisms, human being included, can be inserted into the plasmids of bacteria. The bacteria then proceed to produce the proteins of the foreign species. Movable Genetic Elements When it was first determined that an organism's genes are arranged in sequences on a chromosome, the assumption was made that the position of any one gene would be relatively constant. It was later realized that drastic treatments by chemicals or radiation could cause rearrangements of parts of chromosomes, but these were considered exceptional situations. In 1951, however, U.S. geneticist Barbara McCLINTOCK presented evidence that genetic elements in corn (maize) could change locations among chromosomes.

One of these elements, called the Dissociation locus (Ds), upon insertion next to the gene responsible for pigment production, caused the gene to stop functioning; that is, it acted as the equivalent of a mutation. Any subsequent movement of Ds to another location resulted in the restoration of the pigment-producing gene to its normal function. Almost 20 years later, similar movable elements were discovered in the bacterium Escherichia coli. These genes were usually associated with resistance to antibiotics. The transfer of such a gene involved its movement from its normal site on the bacterium's plasmid to the organism's chromosome, or from the plasmid to the DNA of a virus that happened to be infecting Almost 20 years later, similar movable elements were discovered in the bacterium Escherichia coli. The transfer of such a gene involved its movement from its normal site on the bacterium's plasmid to the organism's chromosome, or from the plasmid to the DNA of a virus that happened to be infecting the bacterial cell.

In 1974, English scientists R.W. Hedges and A.E. Jacob used the word TRANSPOSON to identify any such movable genetic element. Transposon's have also been identified in yeast, protozoa, and fruit flies and are presumed to exist in all organisms, human beings included.

Bibliography

Feder off, Nina V., "Transposable Genetic Elements in Maize", Scientific American, June 1984;
Gardner, E.J., Human Heredity (1983);
Jacob, Francois, The Logic of Life: A History of Heredity, trans. by Betty Spill man (1974;
repr. 1982);
Lewis, K.R., and John, Bernard, The Matter of Mendelian Heredity, 2d ed. (1973);
Newman, Horatio H., et al., Twins: A Study of Heredity and Environment (1966;
Russell, P.J., Genetics, 2d ed. (1990);
Schein feld, Am ram, Your Heredity and Environment (1965);
Winchester, Albert, Heredity, Evolution and Humankind (1976);
Winterton, Bert W., The Processes of Heredity (1980;
repr. 1983).