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THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously © 2007 Paul Billiet ODWSODWS

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Mendel’s peas CharacterTraitAllele Seed shapeRound Wrinkled Cotyledon colourYellow Green © 2007 Paul Billiet ODWSODWS

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Combinations GenotypePhenotype Round Yellow Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWSODWS

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Is the inheritance of one character affected by the inheritance of another? P Phenotypesx(Pure Bred) F 1 PhenotypesAll(Selfed) F 2 Phenotypes Seedcolour YellowGreenTOTALRATIO SeedRound315108423 shapeWrinkled10132133 TOTAL416140556 RATIO © 2007 Paul Billiet ODWSODWS

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A dihybrid cross can be treated as two separate monohybrid crosses The expected probability of each type of seed can be calculated: Probability of an F 2 seed being round = Probability of an F 2 seed being wrinkled = Probability of an F 2 seed being yellow = Probability of an F 2 seed being green = © 2007 Paul Billiet ODWSODWS

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Therefore Probability of an F 2 seed being round and yellow = ¾ x ¾ = 9/16 =56.25% Probability of an F 2 seed being round and green = Probability of an F 2 seed being wrinkled and yellow = Probability of an F 2 seed being wrinkled and green = © 2007 Paul Billiet ODWSODWS

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Predicting how many seeds we could expect to get in a sample We could expect What Mendel observed 556 x 9/16 round yellow313315 556 x 3/16 round green108 556 x 3/16 wrinkled yellow101 556 x 1/16 wrinkled green32 © 2007 Paul Billiet ODWSODWS

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THE LAW OF INDEPENDENT ASSORTMENT It appears that the inheritance of seed shape has no influence over the inheritance of seed colour The two characters are inherited ___________________ The pairs of alleles that control these two characters assort themselves independently © 2007 Paul Billiet ODWSODWS

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Mendel & Meiosis The pairs of chromosomes could orientate in different ways at Anaphase 1 © 2007 Paul Billiet ODWSODWS

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PPhenotypesRound Yellow seed XWrinkled Green seed (Pure bred) GenotypesRRYYrryy meiosis GametesRYry fertilisation F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS

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F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% meiosis GametesYy R r Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS

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GametesYy RRYRy rrYry fertilisation F2F2 GenotypesRYRyrYry RY Ry rY ry Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS

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Dihybrid cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWSODWS

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Dihybrid test cross In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a _____________ This is done with a ___________________ (rr) individual The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for ______ characters (rryy) © 2007 Paul Billiet ODWSODWS

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Dihybrid test cross PhenotypesX Genotypes Gametes GenotypesRYRyrYry PhenotypesRound Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions © 2007 Paul Billiet ODWSODWS

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