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Chapter 14 - Chapter 14 Mendel and the Gene Idea"Blending...

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Chapter 14 Mendel and the Gene Idea
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“Blending ” hypothesis (pre-Mendel) Genetic mtl. contributed by 2 parents mixes in manner analogous to way blue & yellow paints blend to make green “Particulate ” hypothesis of inheritance: the gene idea Parents pass on discrete heritable units, genes
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Gregor Mendel Documented a particulate mechanism of inheritance through expts. with garden peas Figure 14.1
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Concept 14.1: Mendel used scientific approach to identify 2 laws of inheritance 1. Law of Segregation 2. Law of Independent Assortment Mendel chose to work with peas B/c they are available in many varieties B/c he could strictly control which plants mated with which
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Some genetic vocab. Character heritable feature (flower color) Trait variant of a character (purple or white flowers)
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Mendel chose to track Only characters that varied in an “either- or” manner Mendel made sure that Started his experiments with varieties that were “true-breeding” (all offspring are of same variety)
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In typical breeding expt. Mendel mated 2 contrasting, true-breeding varieties (hybridization) True-breeding parents Called P generation
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Hybrid offspring of P generation Called F 1 generation When F 1 individuals self-pollinate F 2 generation is produced
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The Law of Segregation Mendel crossed true-breeding white & purple flowered pea plants All offspring were purple crossed F 1 plants Many had purple flowers, some had white
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Mendel discovered Ratio of about 3 to 1, purple to white flowers, in F 2 generation Figure 14.3 P Generation (true-breeding parents) Purple flowers White flowers × F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by × ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
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Mendel reasoned In F 1 plants, only purple flower factor was affecting flower color in these hybrids Purple flower color was dominant , and white flower color was recessive
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Mendel observed same pattern In other pea plant characters Table 14.1
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Mendel’s Model Mendel developed hypothesis To explain 3:1 inheritance pattern he observed among F 2 offspring 4 related concepts make up this model
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1 - alternative versions of genes Account for variations in inherited characters, now called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
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2 - for each character An organism inherits 2 alleles, one from each parent A genetic locus is actually represented twice
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