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Dihybrid Crosses Biology 30 Mrs. S. Pipke-Painchaud

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Dihybrid Crosses the natural progression for Mendel was to study 2 characteristics at the same time. thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross ex.. round yellow seeds X wrinkled green seeds

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Mendel’s Fourth Law The Law of Independent Assortment During gamete formation, segregating pairs of unit factors assort independently of each other. the two traits are inherited totally independently of each other. i.e. color is inherited independently of seed shape. Example: Homozygous round green X wrinkled homozygous yellow

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Solving Dihybrid Crosses Follow the steps If you were to cross a Homozygous round green X wrinkled homozygous yellow plant, what would your phenotypic and genotypic ratios be?

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Step 1: Make a Key…. G= yellow g = green W = round w = wrinkled

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Step 2: Give the genotypes of the parents... Step 2: Give the genotypes of the parents... Homozygous round green X wrinkled homozygous yellow plant P1 = WWgg X wwGG

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Step 3: Determine the Gametes... WWgg X wwGG WWgg = Wg, Wg, Wg, Wg wwGG = wG, wG, wG, wG HINT: Foil to get the gametes

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Step 4: Fill in the Punnett square... WgWgWgWg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg

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Step 5: Answer any questions... Genotypic ratio = all WwGg Phenotypic ratio = all round yellow G= yellow g = green W = round w = wrinkled

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F1 X F1 = WwGg X WwGg Remember to determine the gametes for your punnett square.

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Fill in the Punnett Square WGWgwGwgWGWWGGWWGgWwGGWwGg WgWWGgWWggWwGgWwgg wGWwGGWwGgwwGGwwGg wgWwGgWwggwwGgwwgg

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The PATTERN... WGWgwGwg WG WWGG WWGg WwGG WwGg Wg WWGg WWgg WwGg Wwgg wG WwGG WwGg wwGG wwGg Wg WwGg Wwgg wwGg wwgg

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What are the ratios? Genotypic Ratio: 1 WWGG 2 WWGg 1 WWgg 2 WwGG 4 WwGg 2 Wwgg 1 wwGG 2 wwGg 1 wwgg WGWgwGwg WGWWGGWWGgWwGGWwGg WgWWGgWWggWwGgWwgg wGWwGGWwGgwwGGwwGg WgWwGgWwggwwGgwwgg

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Phenotypic Ratio: Phenotypic Ratio: 9 Yellow Round Dominant Dominant 3 Yellow Wrinkled Dominant Recessive 3 Green Round Recessive Dominant 1 Green Wrinkled Recessive Recessive Genotypic Ratio: 1 WWGG 2 WWGg 1 WWgg 2 WwGG 4 WwGg 2 Wwgg 1 wwGG 2 wwGg 1 wwgg G= yellow g = green W = round w = wrinkled ** Explain the patterns

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The Short Cut: can only be used if you have (Big little Big little X Big little Big little) BbTt x BbTt Do these fit? BBTt X BbTt BbTt x Bb tt

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Try this: C= Inflated Podc = constricted pod D = Talld = dwarf The cross: a plant heterozygous for inflated pod and heterozygous tall is crossed with a another heterozygous inflated pod and heterozygous tall plant. What are the genotypic and phenotypic ratios? The parents are: CcDd x CcDd

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Example: Does this fit the short cut? Yes C= Inflated Pod c = constricted pod D = Tall d = dwarf Genotypic Ratio: 1 2 1 2 4 2 1 2 1 Genotypic Ratio: 1 CCDD 2 CCDd 1 CCdd 2 CcDD 4 CcDd 2 Ccdd 1 ccDD 2 ccDd 1 ccdd Phenotypic Ratio: (D) (D) 9 (D) ® 3 ® (D) 3 ® ® 1 Phenotypic Ratio: (D) (D) 9 Inflated Tall (D) ® 3 Inflated Dwarf ® (D) 3 Constricted Tall ® ® 1 Constricted Dwarf

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Now try... Refer to the assigned questions in your duotang.

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