IB31Lecture+5+outlineMendelian+genetics+Sp08

IB31Lecture+5+outlineMendelian+genetics+Sp08 - I.B 31...

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I.B. 31, Spring, 2008 Feb. 11, 2008 Basic Mendelian Genetics Lecture This is a “background” lecture, so if you remember your basic genetics, sit back and relax – it will be over soon! Through much of human history, the idea has been prevalent that behavioral traits, like physical ones, are inherited. Certainly the domestication of dogs 14,000 years ago depended on this idea whether or not it was overtly expressed. Even today, we use terms like "a chip off of the old block", "like father, like son", and "it runs in the family". Tell story of Bartolommeo de Vinci, the 45 year younger half sib of Leonardo. Leonardo was the son of their father, Piero, a notary, and Caterina, a peasant girl from Vinci. Bartolommeo, also a notary, went back to Vinci, found a peasant girl who matched the description of Caterina, and married her. Their son, also named Piero, was a skilled artist, but died young. I. DARWIN. Darwin included in his Theory of Natural Selection the premise that phenotypic traits varied and that variation had a heritable component. He recognized two kinds of variation: A. Continuous Variation - small differences from individual to individual. B. Sports - occasional major variants (albinos, crumpled wings, etc.) Both were known to be inherited (from domestication studies), but Darwin thought that "sports" were too rare to play a major role in evolution. He accepted the prevalent view of "Blending Inheritance": traits of offspring tended to be the average of the traits of the parents. He recognized that interbreeding would eliminate variation. This is one reason that he stressed geographical isolation in speciation. Isolation prevented interbreeding throughout the entire species and allowed natural selection to accumulate small differences in local populations. He could not explain the source of heritable variation. "Pangenesis" was the best he could come up with. This was the Larmarkian idea that somatic cells contained gemmules that were influenced by the environment and by organ use and disuse (eyeless cave species of fish). Gemmules then moved to sex
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cells and were incorporated into eggs and sperm. They in turn, influenced somatic cells in the new organism. II. At the same time, Gregor Mendel, an Augustinian Monk was working in Brunn, Moravia. He was interested in how traits were inherited and in the 1850's and 60's he studied how this occurred in plants. He first tried experiments on honey bees, but the Brothers objected, so he switched to peas. He published his results in 1865. III. Lets quickly review one of Mendel's basic breeding experiments using peas. He developed seven true-breeding strains that each varied with respect to one particular trait. For example, seed coat, smooth and wrinkled. If he bred smooth to smooth, he always got smooth; wrinkled to wrinkled gave wrinkled.
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