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Download Mol Bio CH1 Sept 13
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Gregor Mendel -Presented work on heredity in 1865 -Attempted to demonstrate the usefulness of laws of heredity for evolution, was ignored Mendel’s first experiment on Heredity - Do Peas “Breed True” Yes. Traits are inherited from parents to offspring - Why was this necessary? Mendel’s second experiment: -Bread varieties with different traits -Carried out breeding for multiple generations (F1, F2…) -Found that one trait was lost during F1 generation, but returned in lower numbers in F2 generation 3:1 (75%:25%) -Concluded that each trait has 2 “factors” -Concluded that each “factor” segregates independently during formation of sex cells (1st law of heredity) QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this pi cture. Dominance is not required - Independent segregation still explains heredity when traits “blend” QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Mendel’s third experiment: -Bread varieties with multiple alternative traits -Observed 9:3:3:1 ratio, including new combinations of traits QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this pi cture. -Concluded that traits are independently assorted. 2nd law of heredity. -(This law isn’t always correct, and probably shouldn’t be called a law) Mendel was ignored because the mechanism of heredity was unknown -Darwin’s black box was Mendel’s black box too -Mendel’s results were replicated by 3 others in 1900, geneticists started taking Mendel seriously -Walter Sutton published “The chromosomes in heredity” in 1903 “Gene Linkage” occurs when two genes reside on the same chromosome - violates the 2nd law of heredity - Gene linkage can be broken by “crossing over” during meiosis QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Physical proof of crossing over -Even before chromosomes were firmly established as molecules of heredity, crossing over had been demonstrated (1931) QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. -Diagram shows the results of no crossing over and crossing over when individual chromosomes can be identified Chromosome mapping -Morgan and undergrads looked at “mutations” in fruit flies QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. -Could use the frequency of “recombinants” (broken linkage groups) to estimate the relative positions of genes on a chromosome QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. The larger the distance, the more likely a cross over even will occur QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. -Morgan et al. helped establish that genes reside on chromosomes. Could map the relative distance of genes on a chromosome Could explain all of Mendel’s results with genetics (Morgan) and cytology (Sutton) -Morgan was not taken seriously by classic biologists. Where do mutations come from? Are “fruit fly” mutations significant enough for large scale evolution? What are genes? - DNA or Protein? (Chromosomes are made of both) - Genes must be able to self replicate to allow heredity Various observations suggested that genes are correlated to with enzymes…….. ……..Do genes control enzymes, are enzymes synthesized by genes? By the mid 1900’s biologist could say” -Genes segregate independently in sex cells -Genes reside on chromosomes -Give a relative position to genes on chromosomes -Could explain much of Darwin’s black box by genes on chrom., inherited by offspring with crossing over, mutation