THE DIHYBRID CROSS

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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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Mendels 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 Round Yellow
RrYY Round Yellow
RrYy Round Yellow
RRyy Round Green
Rryy Round Green
rrYY Wrinkled Yellow
rrYy Wrinkled Yellow
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
Seed Round 315 108 423
shape Wrinkled 101 32 133
TOTAL 416 140 556
RATIO
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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
Seed Round 315 108 423 3.18
shape Wrinkled 101 32 133 1
TOTAL 416 140 556
RATIO 2.97 1
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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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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 56.25
• 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 56.25
• Probability of an F2 seed being round and green
  ¾ x ¼ 3/16 18.75
• Probability of an F2 seed being wrinkled and
  yellow ¼ x ¾ 3/16 18.75
• Probability of an F2 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 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
We could expect 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 104 101
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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Dihybrid cross genetic diagram
P Phenotypes Round Yellow seed Round Yellow seed X X Wrinkled Green seed Wrinkled Green seed Wrinkled Green seed (Pure bred)
Genotypes RRYY RRYY rryy rryy rryy
meiosis meiosis meiosis meiosis meiosis

Gametes RY RY ry ry ry
fertilisation fertilisation fertilisation
F1 Phenotypes RrYy RrYy (Selfed)
Genotypes Round Yellow Round Yellow Round Yellow Round Yellow Round Yellow
Proportions 100
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Dihybrid cross genetic diagram
F1 Phenotypes RrYy (Selfed)
Genotypes Round Yellow Round Yellow Round Yellow Round Yellow Round Yellow
Proportions 100 100 100
meiosis meiosis meiosis
Gametes Y Y Y y
R RY RY RY Ry
r rY rY rY ry
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Dihybrid cross genetic diagram
Gametes Y y
R RY Ry
r rY ry
fertilisation
F2 Genotypes RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy