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Genetics And Heredity
Heredity is the passing on of characteristics from parents to offspring. All living things pass on traits from one generation to the next. All children inherit traits from their parents. A boy may be blond and blue-eyed like his mother. A girl may have curly hair like her father , and a nose like her grandmother. Sometimes traits can be traced to more distant relatives. The traits that one inherits from parents by way of the sperm and egg are called biological inheritance. All inherited traits may not stay the same throughout a lifetime. Studies of identical twins, who have the same biological inheritance, show that environment can cause changes in physical traits. If one twin had a serious disease or was accidentally injured, his/her appearance might change. He/she would then look different from his twin brother. Every person inherits his basic mental ability which determines his/her capacity to learn. Environment gives him/her opportunities to develop this capacity. The amount a person can learn depends both on his/her inherited ability and on his/her environment -- parents, community, and school. The cell contains bodies that determine which traits are inherited. These bodies are called chromosomes and genes. The chromosomes carry the genes. Every human body cell contains two sets of 23 chromosomes. These two sets look very much alike. Each chromosomes in one set can be matched with a particular chromosome in the other set. Egg cells and sperm cells have only one set of 23 chromosomes. As a result when an egg and a sperm come together, the fertilized egg cell will contain the 46 chromosomes of a normal body cell. Half of the chromosomes come from the mother, and half from the father. Whether a fertilized egg cell develops into a girl or a boy is determined by two special chromosomes called sex chromosomes. In girls, the two sex chromosomes look exactly alike. Scientists call them X chromosomes. In boys, the two chromosomes do not look alike. One of them is an X chromosome, but the other is shorter and does not match with its partner. Scientists call this shorter chromosome a Y chromosome, Genes are tiny particles that determine all hereditary traits. They are carried on the chromosomes. Each chromosome bears hundreds of genes, arranged in a line along its length. Every person inherits two genes for a particular trait -- one from his/her mother and one from his/her father. But in many traits, the action of one gene completely overpowers the action of the other. The powerful gene is called dominant and the other recessive. For example, suppose a dark-haired father and a red-haired mother have a dark-haired child. In this case, each of the child's cells contains a gene for dark hair and a gene for red hair. But because the child has dark hair, it is apparent that the gene for dark hair is dominant over the gene for red hair. Recessive genes do not visibly show up when there is a dominant gene present. A hybrid is a person who has both a dominant and recessive gene. Their appearance is that of a person who has no recessive gene, but they carry the recessive gene, and it is possible their children will also. There are also times when there is a compromise. If two genes are both dominant, the resulting offspring will have a blend of those genes. For example, a black rooster and a white hen mate, and the resulting offspring are gray. The same can be said for people. Genes are made of deoxyribonucleic acid, or DNA for short. DNA is a threadlike molecule with a diameter of only about one ten-millionth of an inch. The thread is actually double, and resembles a tiny twisted rope ladder. A DNA molecule contains two kinds of purine bases. They are adenine (called A) and guanine )called G). It also contains two kinds of pyrimidine bases called cytosine (C) and thymine (T). A and T fit together and C and G fit together. No other combinations work. In other words, there are only four kind of base-pair combinations in a DNA ladder: A-T and C-G, or T-A and G-C. All genes in every cell contain these same four bases, A, T, C, and G. The difference between one gene and another lies in the arrangement of pairs of bases along the DNA molecule. There are at least several hundred pairs of bases in each gene. For this reason, a vast number of arrangements is possible, and countless kinds of genes can be formed. Another aspect of genetics is mutation. A gene is an extremely stable structure. It may remain stable through thousands of generations, copying itself correctly in between each cell division. Occasionally, however, a mistake occurs in the copying process. This mistake may upset the ordered sequence of base-pairs in a gene and cause a mutation. Some mutations are helpful and may change an animal or plant so that it is better equipped to live in its surroundings. For example, once ducks had no webbing on their feet. Only mutant ducks had webbing at the time. The mutant ducks were better swimmers, so they survived attacks from predators while ducks without webbing did not. Thus only ducks with webbed feet survived, and we have the modern duck. Man's understanding of heredity dates to ancient times. A 6,000 year old engraved stone found in Chaldea, near the Persian Gulf, traced various traits through five generations of horses. But ancient men did not know how or why the principles of heredity worked. Early Greek philosophers including Aristotle thought blood was the basic element of heredity. Such phrases as blood line, blue blood, and blood relative show the influence of Aristotle's ideas. The 1800's brought the first theories about how traits change from generation to generation. Gregor Mendel, an Austrian monk, made the greatest single contribution to the study of heredity and founded the science of genetics. By careful experiment, observation, and analysis of his results, Mendel formulated the first laws of heredity. Bibliography: Bailey, Marilyn. Evolution: Opposing Viewpoints. Greenhaven Press, Inc. San Diego California, 1990. Bernstein, Vernstein, Martin Schachter, Alan Winkler, and Stanley Wolfe. Concepts and Challenges in Life Science, Second Edition. Allyn and Bacon Inc. Newton Massachusetts, 1986. Coryllel Lipke, Jean. Heredity. Lerner Publications Company, United States, 1971.


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