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THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously © 2007 Paul Billiet ODWSODWS
Mendel’s peas CharacterTraitAllele Seed shapeRound Wrinkled Cotyledon colourYellow Green © 2007 Paul Billiet ODWSODWS
Combinations GenotypePhenotype Round Yellow Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWSODWS
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 SeedRound shapeWrinkled TOTAL RATIO © 2007 Paul Billiet ODWSODWS
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
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
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 © 2007 Paul Billiet ODWSODWS
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
Mendel & Meiosis The pairs of chromosomes could orientate in different ways at Anaphase 1 © 2007 Paul Billiet ODWSODWS
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
F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% meiosis GametesYy R r Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS
GametesYy RRYRy rrYry fertilisation F2F2 GenotypesRYRyrYry RY Ry rY ry Dihybrid cross genetic diagram © 2007 Paul Billiet ODWSODWS
Dihybrid cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWSODWS
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
Dihybrid test cross PhenotypesX Genotypes Gametes GenotypesRYRyrYry PhenotypesRound Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions © 2007 Paul Billiet ODWSODWS
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