Wk 5 Lec 2

Wk 5 Lec 2 - Whatwerewetalkingabout?...

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What were we talking about? Introduction to field of genetics Historic ideas: pangenesis, blending Mendel Experimental design Monohybrid Cross Ratios Punnett Squares
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Chapter 9 Patterns of Inheritance
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Principle of Independent Assortment Two of the seven characteristics Mendel studied were  seed shape and seed color  Mendel’s seeds were either round or wrinkled in shape,  and they were either yellow or green in color  From monohybrid crosses, Mendel knew that the allele  for round shape (designated R) was dominant to the  allele for wrinkled shape (r) and the allele for yellow  seed color (Y) was dominant to the allele for green seed  color (y) What would result from a mating of parental varieties  differing in two characteristics - a dihybrid cross?
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Dihybrid Cross Mendel crossed homozygous plants having round  yellow seeds (genotype RRYY) with plants having  wrinkled green seeds (rryy).   The union of RY and ry gametes yielded hybrids  heterozygous for both characteristics (RrYy) – that is,  dihybrids All of these offspring, the F 1  generation, had round  yellow seeds But were the two characteristics transmitted from  parents to offspring as a package, or was each  characteristic inherited independently of the other? 
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Dihybrid Cross The question was answered when Mendel allowed  fertilization to occur among the F 1  plants If the genes for the two characteristics were inherited  together, then the F 1  hybrids would produce only the same  two kinds (genotypes)  of gametes that they received from  their parents  The F2 generation would show a 3:1 phenotypic ratio (three  plants with round yellow seeds for every one with wrinkled  green seeds), as in a Punnett Square  If however, the two seed characteristics segregated  independently, then the F 1  generation would produce four  gamete genotypes – RY, rY, Ry, and ry – in equal quantities
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Dihybrid Cross The Punnett Square from this experiment shows  that there are nine different genotypes in the F 2 However, there are only four kinds of phenotypes,  with a ratio of 9:3:3:1 The Punnett square also reveals that a dihybrid  cross is equivalent to two monohybrid crosses  occurring simultaneously  There are 12 plants with round  seeds to 4 with  wrinkled  seeds and 12 yellow-seeded  plants to 4  green-seeded  ones These 12:4 ratios each reduce to 3:1, which is the F 2   ratio for a monohybrid cross
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Independent Assortment Mendel tried his seven pea  characteristics in various dihybrid  combinations and always observed a 
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Wk 5 Lec 2 - Whatwerewetalkingabout?...

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