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Studying the inheritance of two characters simultaneously
THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously © 2016 Paul Billiet ODWS
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Mendel’s peas Character Trait Allele Seed shape Round R Wrinkled r
Cotyledon colour Yellow Y Green y © 2016 Paul Billiet ODWS
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Combinations Genotype Phenotype RRYY Round Yellow RRYy RrYY RrYy RRyy
Round Green Rryy rrYY Wrinkled Yellow rrYy rryy Wrinkled Green © 2016 Paul Billiet ODWS
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Is the inheritance of one character affected by the inheritance of another?
P Phenotypes Round Yellow x Wrinkled Green (Pure Bred) F1 Phenotypes All Round Yellow (Selfed) F2 Phenotypes Seed colour Yellow Green TOTAL RATIO Round 315 108 423 shape Wrinkled 101 32 133 416 140 556 © 2016 Paul Billiet ODWS
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Is the inheritance of one character affected by the inheritance of another?
P Phenotypes Round Yellow x Wrinkled Green (Pure Bred) F1 Phenotypes All Round Yellow (Selfed) F2 Phenotypes Seed colour Yellow Green TOTAL RATIO Round 315 108 423 3.18 shape Wrinkled 101 32 133 1 416 140 556 2.97 © 2016 Paul Billiet ODWS
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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 F2 seed being round = 75% or 3/4 Probability of an F2 seed being wrinkled = Probability of an F2 seed being yellow = Probability of an F2 seed being green = © 2016 Paul Billiet ODWS
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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 F2 seed being round = 75% or 3/4 Probability of an F2 seed being wrinkled = 25% or 1/4 Probability of an F2 seed being yellow = 75% or 3/4 Probability of an F2 seed being green = 25% or 1/4 © 2016 Paul Billiet ODWS
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Therefore Probability of an F2 seed being round and yellow = 3/4 x 3/4 = 9/16 = % Probability of an F2 seed being round and green = Probability of an F2 seed being wrinkled and yellow = Probability of an F2 seed being wrinkled and green = © 2016 Paul Billiet ODWS
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Therefore Probability of an F2 seed being round and yellow = 3/4 x 3/4 = 9/16 = % Probability of an F2 seed being round and green = 3/4 x 1/4 = 3/16 = % Probability of an F2 seed being wrinkled and yellow = 1/4 x 3/4 = 3/16 = % Probability of an F2 seed being wrinkled and green = 1/4 x 1/4 =1/16 = % © 2016 Paul Billiet ODWS
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We could expect What Mendel observed 556 x 9/16 round yellow 313 315
Predicting how many seeds we could expect to get in a sample We could expect What Mendel observed 556 x 9/16 round yellow 313 315 556 x 3/16 round green 108 556 x 3/16 wrinkled yellow 101 556 x 1/16 wrinkled green 32 © 2016 Paul Billiet ODWS
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Predicting how many seeds we could expect to get in a sample
What Mendel observed 556 x 9/16 round yellow 313 315 556 x 3/16 round green 104 108 556 x 3/16 wrinkled yellow 101 556 x 1/16 wrinkled green 35 32 © 2016 Paul Billiet ODWS
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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 © 2016 Paul Billiet ODWS
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Mendel & Meiosis The pairs of chromosomes could orientate in different ways at Anaphase 1 © 2016 Paul Billiet ODWS
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Dihybrid cross genetic diagram
P Phenotypes Round Yellow seed X Wrinkled Green seed (Pure bred) Genotypes RRYY rryy meiosis Gametes RY ry fertilisation F1 RrYy (Selfed) Round Yellow Proportions 100% © 2016 Paul Billiet ODWS
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Dihybrid cross genetic diagram
F1 Genotypes RrYy (Selfed) Phenotypes Round Yellow Proportions 100% meiosis Gametes Y y R RY Ry r rY ry © 2016 Paul Billiet ODWS
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Dihybrid cross genetic diagram
Gametes Y y R RY Ry r rY ry fertilisation F2 Genotypes RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy © 2016 Paul Billiet ODWS
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Dihybrid cross proportions
Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green Wrinkled Yellow Wrinkled Green © 2016 Paul Billiet ODWS
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Dihybrid cross proportions Phenotypes Proportions Round Yellow 9/16 or
56.25% Round Green 3/16 or 18.75% Wrinkled Yellow Wrinkled Green 1/16 or 6.25% © 2016 Paul Billiet ODWS
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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 using 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). © 2016 Paul Billiet ODWS
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Dihybrid test cross Phenotypes Round Yellow X Wrinkled Green Genotypes
RrYy rryy Gametes RY, Ry, rY, ry ry Genotypes RY Ry rY ry RrYy Rryy rrYy rryy Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions 1/4 or 25% © 2016 Paul Billiet ODWS
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