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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 shapeRoundR Wrinkledr Cotyledon colourYellowY Greeny © 2007 Paul Billiet ODWSODWS

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Combinations GenotypePhenotype RRYYRound Yellow RRYyRound Yellow RrYYRound Yellow RrYyRound Yellow RRyyRound Green RryyRound Green rrYYWrinkled Yellow rrYyWrinkled Yellow rryyWrinkled Green © 2007 Paul Billiet ODWSODWS

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

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Is the inheritance of one character affected by the inheritance of another? P PhenotypesRound Yellow xWrinkled Green (Pure Bred) F 1 PhenotypesAll Round Yellow (Selfed) F 2 Phenotypes Seedcolour YellowGreenTOTALRATIO SeedRound3151084233.18 shapeWrinkled101321331 TOTAL416140556 RATIO2.971 © 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 = 75% or ¾ 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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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 = 75% or ¾ Probability of an F 2 seed being wrinkled = 25% or ¼ Probability of an F 2 seed being yellow = 75% or ¾ Probability of an F 2 seed being green = 25% or ¼ © 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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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 = ¾ x ¼ = 3/16 =18.75% Probability of an F 2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 =18.75% Probability of an F 2 seed being wrinkled and green = ¼ x ¼ =1/16 =6.25% © 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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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 green104108 556 x 3/16 wrinkled yellow104101 556 x 1/16 wrinkled green3532 © 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 INDEPENDENTLY 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 RRYRy rrYry Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS

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GametesYy RRYRy rrYry fertilisation F2F2 GenotypesRYRyrYry RYRRYYRRYyRrYYRrYy RyRRYyRRyyRrYyRryy rYRrYYRrYyrrYYrrYy ryRrYyRryyrrYyrryy 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 cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green3/16 or18.75% Wrinkled Yellow3/16 or18.75% Wrinkled Green1/16 or6.25% © 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 test cross This is done with a homozygous recessive (rr) individual The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy) © 2007 Paul Billiet ODWSODWS

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Dihybrid test cross PhenotypesRound YellowXWrinkled Green GenotypesRrYyrryy GametesRY, Ry, rY, ryry GenotypesRYRyrYry RrYyRryyrrYyrryy PhenotypesRound Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions1/4 or 25% © 2007 Paul Billiet ODWSODWS

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