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CHAPTER 14 MENDEL AND THE GENE IDEA I. Gregor Mendel's Discoveries Based upon their observations from ornamental plant breeding, biologists in the 19th century realized that both parents contribute to the characteristics of offspring. Before Mendel, the favored explanation of heredity was the blending theory. Blending theory of heredity = Pre-Mendelian theory of heredity proposing that hereditary material from each parent mixes in the offspring; once blended like two liquids in solution, the hereditary material is inseparable and the offspring's traits are some intermediate between the parental types. According to this theory: - Individuals of a population should reach a uniform appearance after many generations. - Once hereditary traits are blended, they can no longer be separated out to appear again in later generations. This blending theory of heredity was inconsistent with the observations that: - Individuals in a population do not reach a uniform appearance; inheritable variation among individuals is generally preserved. - Some inheritable traits skip one generation only to reappear in the next. Modern genetics began in the 1860s when Gregor Mendel , an Augustinian monk, discovered the fundamental principles of heredity. Mendel's great contribution to modern genetics was to replace the blending theory of heredity with the particulate theory of heredity. Particulate theory of heredity = Gregor Mendel's theory that parents transmit to their offspring discrete inheritable factors (now called genes) that remain as separate factors from one generation to the next. A. Mendel brought an experimental and quantitative approach to genetics: In 1857, Mendel was living in an Augustinian monastery, where he bred garden peas in the abbey garden. He probably chose garden peas as his experimental organisms
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because: - They were available in many easily distinguishable varieties. - Strict control over mating was possible to ensure the parentage of new seeds. Petals of the pea flower enclose the pistil and stamens, which prevents cross pollination. Immature stamens can be removed to prevent self-pollination. Mendel hybridized pea plants by transferring pollen from one flower to another with an artist's brush. Character = Detectable inheritable feature of an organism Trait = Variant of an inheritable character Mendel chose characters in pea plants that differed in a relatively clear-cut manner. He chose seven characters, each of which occurred in two alternative forms: 1. Flower color (purple or white) 2. Flower position (axial or terminal) 3. Seed color (yellow or green) 4. Seed shape (round or wrinkled) 5. Pod shape (inflated or constricted) 6. Pod color (green or yellow) 7. Stem length (tall or dwarf) True breeding = Always producing offspring with the same traits as the parents when the parents are self-fertilized Mendel started his experiments with true- breeding plant varieties, which he hybridized (cross-pollinated) in experimental crosses. - The true-breeding parental plants of such a cross are called the P
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This note was uploaded on 03/18/2008 for the course BS 110 taught by Professor S.lawrence during the Spring '07 term at Michigan State University.

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