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THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously BY- DR (Mrs.) T.S.KHAN (PGT BIOLOGY)

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M ENDEL ’ S PEAS CharacterTraitAllele Seed shapeRoundR Wrinkledr Cotyledon colourYellowY Greeny

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C OMBINATIONS GenotypePhenotype RRYYRound Yellow RRYyRound Yellow RrYYRound Yellow RrYyRound Yellow RRyyRound Green RryyRound Green rrYYWrinkled Yellow rrYyWrinkled Yellow rryyWrinkled Green

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I S 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 SeedRound shapeWrinkled TOTAL RATIO

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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 SeedRound shapeWrinkled TOTAL RATIO2.971

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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 =

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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

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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 =

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T HEREFORE 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%

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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 yellow x 3/16 round green x 3/16 wrinkled yellow x 1/16 wrinkled green32

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P REDICTING HOW MANY SEEDS WE COULD EXPECT TO GET IN A SAMPLE We could expect What Mendel observed 556 x 9/16 round yellow x 3/16 round green x 3/16 wrinkled yellow x 1/16 wrinkled green3532

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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

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PPhenotypesRound Yellow seed XWrinkled Green seed (Pure bred) GenotypesRRYYrryy meiosis GametesRYry fertilisation F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% D IHYBRID CROSS GENETIC DIAGRAM

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F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% meiosis GametesYy RRYRy rrYry D IHYBRID CROSS GENETIC DIAGRAM

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GametesYy RRYRy rrYry fertilisation F2F2 GenotypesRYRyrYry RYRRYYRRYyRrYYRrYy RyRRYyRRyyRrYyRryy rYRrYYRrYyrrYYrrYy ryRrYyRryyrrYyrryy D IHYBRID CROSS GENETIC DIAGRAM

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