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Studying the inheritance of two characters simultaneously SBI3U
THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously SBI3U
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Independent Assortment
Are Different Characters Like Color and Shape Inherited Together or Inherited Independently? Mendel performed dihybrid crosses to find out. Mendel’s conclusion: Different characters are inherited independently.
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Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another?
The alternative and incorrect hypothesis: dependent inheritance. Because it’s the only way to explain the pattern of inheritance.
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The Punnett Square Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.
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The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment The alignment of one pair of homologs is independent of any other. Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.
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Peas & Humans Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.
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Mendel’s peas Character Trait Allele Seed shape Round R Wrinkled r
Cotyledon colour Yellow Y Green y
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Combinations Genotype Phenotype RRYY Round Yellow RRYy RrYY RrYy RRyy
Round Green Rryy rrYY Wrinkled Yellow rrYy rryy Wrinkled Green
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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
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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
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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 ¾ Probability of an F2 seed being wrinkled = Probability of an F2 seed being yellow = Probability of an F2 seed being green =
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The expected probability of each type of seed can be calculated:
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 ¾ Probability of an F2 seed being wrinkled = 25% or ¼ Probability of an F2 seed being yellow = 75% or ¾ Probability of an F2 seed being green = 25% or ¼
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Therefore Probability of an F2 seed being round and yellow = ¾ x ¾ = 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 =
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Therefore Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = % Probability of an F2 seed being round and green = ¾ x ¼ = 3/16 = % Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = % Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16 = %
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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
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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
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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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Mendel & Meiosis The pairs of chromosomes could orientate in different ways at Anaphase 1
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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%
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Dihybrid cross genetic diagram
F1 Phenotypes RrYy (Selfed) Genotypes Round Yellow Proportions 100% meiosis Gametes Y y R RY Ry r rY ry
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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
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Dihybrid cross proportions
Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green Wrinkled Yellow Wrinkled Green
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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%
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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)
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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%
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