Dihybrid Notes

1
Dihybrid Notes

• Predicting crosses involving two traits

2

• Individuals have many genes, so genetics problems
  can be done to determine how more than one trait
  is inherited at a time. In this note packet, we
  are going to focus on crosses involving the
  inheritance of two traits at once.
• When you write out the genotype of an individual,
  there are certain rules to follow.

3

• When you write out the genotype of an individual,
  there are certain rules to follow.
• In the genotype, list the genes in the order
  given in the problem.
• Ex. Long fur, L, is dominant to short fur, l.
  Striped fur, F, is dominant to solid fur, f. A
  genotype of an individual that is homozygous for
  both fur length and pattern is LLFF.
• Always list the dominant allele first.
• Ex. LlFf is correct while lLfF is not.

4
Give it a try

• In pea plants, long stems, L, are dominant to
  short stems, l. Green pea pods, G, are dominant
  to yellow pea pods, g.
• Record the phenotype of the following individuals
  based on their genotypes. The first one is done
  for you as an example.
• LlGg The plant has long stems and green pods.

5

1. LLGG
2. llGg
3. llgg

6

1. LLGG The plant has long stems and green
   pods.
2. llGg
3. llgg

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1. LLGG The plant has long stems and green
   pods.
2. llGg The plant has short stems and green pods.
   
3. llgg

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1. LLGG The plant has long stems and green
   pods.
2. llGg The plant has short stems and green pods.
   
3. llgg The plant has short stems and yellow pods
   

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• E. LlGG
• F. LLgg
• G. LLGG

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• E. LlGG The plant has long stems and green
  pods
• F. LLgg
• G. LLGG

11

• E. LlGG The plant has long stems and green
  pods
• F. LLgg The plant has long stems and yellow
  pods
• G. LLGG

12

• E. LlGG The plant has long stems and green
  pods
• F. LLgg The plant has long stems and yellow
  pods
• G. LLGG The plant has long stems and green
  pods

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Set 2

• Record the genotype of the following individuals
  based on their descriptions. The first one is
  done for you as an example.
• An individual that is heterozygous for long stems
  and has yellow pods Llgg

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• An individual that has short stems and is
  homozygous dominant for green pods
• An individual that is heterozygous for both stem
  length and pod color
• An individual that is homozygous for long stems
  and has yellow pods

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• An individual that has short stems and is
  homozygous dominant for green pods llGg
• An individual that is heterozygous for both stem
  length and pod color
• An individual that is homozygous for long stems
  and has yellow pods

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• An individual that has short stems and is
  homozygous dominant for green pods llGg
• An individual that is heterozygous for both stem
  length and pod color LlGg
• An individual that is homozygous for long stems
  and has yellow pods

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• An individual that has short stems and is
  homozygous dominant for green pods llGg
• An individual that is heterozygous for both stem
  length and pod color LlGg
• An individual that is homozygous for long stems
  and has yellow pods LLgg

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1. An individual that has a short stem and yellow
   pods
2. An individual that is homozygous for both long
   stems and green pods
3. An individual that is pure breeding for both long
   stems and yellow pods

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1. An individual that has a short stem and yellow
   pods llgg
2. An individual that is homozygous for both long
   stems and green pods
3. An individual that is pure breeding for both long
   stems and yellow pods

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1. An individual that has a short stem and yellow
   pods llgg
2. An individual that is homozygous for both long
   stems and green pods LLGG
3. An individual that is pure breeding for both long
   stems and yellow pods

21

1. An individual that has a short stem and yellow
   pods llgg
2. An individual that is homozygous for both long
   stems and green pods LLGG
3. An individual that is pure breeding for both long
   stems and yellow pods LLgg

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What gametes are formed in dihybrid crosses?
23
What gametes are formed in dihybrid crosses?

• Mendels Law of Independent Assortment

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What gametes are formed in dihybrid crosses?

• Mendels Law of Independent Assortment Genes
  that segregate independently do not influence
  each others inheritance

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What gametes are formed in dihybrid crosses?

• Mendels Law of Independent Assortment Genes
  that segregate independently do not influence
  each others inheritance
• When there are two genes, each gamete will have
  one allele for each gene. There are four
  possible genetic combinations for the gametes.

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Possibility 1 for AaBb
A
a
Parent Cell AaBb
B
b
Meiosis I
a
A
b
B
Meiosis II
A
a
a
A
B
b
B
b
Gametes
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Possibility 2 for AaBb
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Give it a try

• What 4 gametes would an individual with the
  genotype AaBb make?
• What 4 gametes would an individual with the
  genotype TtMM make?

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Give it a try

• What 4 gametes would an individual with the
  genotype AaBb make?
• AB, Ab, aB, ab
• What 4 gametes would an individual with the
  genotype TtMM make?

30
Give it a try

• What 4 gametes would an individual with the
  genotype AaBb make?
• AB, Ab, aB, ab
• What 4 gametes would an individual with the
  genotype TtMM make?
• TM, TM, tM, tM

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Remember

• Gametes cant have two copies of the same gene
  (same letter), because one copy of the gene is
  supposed to come from each parent in each gamete.
  
• If you have two of the same genes in a gamete,
  nondisjunction occurred and the resulting baby
  from that gamete would have an extra chromosome
  and some diseases.

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Crosses Involving 2 Traits or Dihybrid Crosses

• Llong stems and lshort stems
• Ggreen pods and gyellow pods
• Cross GgLl x GgLl
• What will the gametes look like?

GL
Gl
gl
gL
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GL
Gl
gL
gl




GgLl
GGLL
GGLl
GgLL
GL
GgLl
GGLl
GGll
Ggll
Gl
gL
GgLL
GgLl
ggLL
ggLl
gl
GgLl
Ggll
ggll
ggLl
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• Possible Phenotypes
• Green pods with long stems
• Green pods with short stems
• Yellow pods with long stems
• Yellow pods with short stems
• Phenotypic ratio
• 9331

9
3
3
1
35
Dihybrid problems
36
1. What kinds of gametes would be produced by
organisms having the following genotypes?

• aaBB
• AaBB
• AaBb
• Aabb

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1. What kinds of gametes would be produced by
organisms having the following genotypes?

• aaBB aB, aB, aB, aB
• AaBB
• AaBb
• Aabb

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1. What kinds of gametes would be produced by
organisms having the following genotypes?

• aaBB aB, aB, aB, aB
• AaBB AB, AB, aB, aB
• AaBb
• Aabb

39
1. What kinds of gametes would be produced by
organisms having the following genotypes?

• aaBB aB, aB, aB, aB
• AaBB AB, AB, aB, aB
• AaBb AB, Ab, aB, ab
• Aabb

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1. What kinds of gametes would be produced by
organisms having the following genotypes?

• aaBB aB, aB, aB, aB
• AaBB AB, AB, aB, aB
• AaBb AB, Ab, aB, ab
• Aabb Ab, Ab, ab, ab

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• 2. In dogs, the allele F results in a dog that
  fetches and the allele f results in a dog that
  will not fetch. The allele D results in a dog
  that sits on command, while the allele d results
  in a dog that does not sit on command. You cross
  a male and female that both fetch and sit, and
  have the genotypes FfDd and FfDd.

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FD
Fd
fD
fd
A.




FfDd
FFDD
FFDd
FfDD
FD
FfDd
FFDd
FFdd
Ffdd
Fd
fD
FfDD
FfDd
ffDD
ffDd
fd
FfDd
Ffdd
ffdd
ffDd
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• What is the phenotypic ratio of the offspring? 
  
• What fraction of the offspring will neither fetch
  nor sit?
• What fraction of the offspring will be FFdd? 
  
• What fraction of the offspring will fetch, but
  not sit on command?

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• What is the phenotypic ratio of the offspring? 
  9331
• What fraction of the offspring will neither fetch
  nor sit?
• What fraction of the offspring will be FFdd? 
  
• What fraction of the offspring will fetch, but
  not sit on command?

45

• What is the phenotypic ratio of the offspring? 
  9331
• What fraction of the offspring will neither fetch
  nor sit? 1/16
• What fraction of the offspring will be FFdd? 
  
• What fraction of the offspring will fetch, but
  not sit on command?

46

• What is the phenotypic ratio of the offspring? 
  9331
• What fraction of the offspring will neither fetch
  nor sit? 1/16
• What fraction of the offspring will be FFdd? 
  1/16
• What fraction of the offspring will fetch, but
  not sit on command?

47

1. What is the phenotypic ratio of the offspring? 
   9331
2. What fraction of the offspring will neither fetch
   nor sit? 1/16
3. What fraction of the offspring will be FFdd? 
   1/16
4. What fraction of the offspring will fetch, but
   not sit on command? 3/16

48

• In peas, tall plants (T) are dominant to short
  plants (t). Round seeds (R) are dominant to
  wrinkled seeds (r). Show a cross between a plant
  with the genotype TtRr and a plant with the
  genotype TTRR.

49

• In peas, tall plants (T) are dominant to short
  plants (t). Round seeds (R) are dominant to
  wrinkled seeds (r). Show a cross between a plant
  with the genotype TtRr and a plant with the
  genotype TTRR.
• Possible gametes from TtRr

50

• In peas, tall plants (T) are dominant to short
  plants (t). Round seeds (R) are dominant to
  wrinkled seeds (r). Show a cross between a plant
  with the genotype TtRr and a plant with the
  genotype TTRR.
• Possible gametes from TtRr TR, Tr, tR, tr

51

• In peas, tall plants (T) are dominant to short
  plants (t). Round seeds (R) are dominant to
  wrinkled seeds (r). Show a cross between a plant
  with the genotype TtRr and a plant with the
  genotype TTRR.
• Possible gametes from TtRr TR, Tr, tR, tr
• Possible gametes from TTRR

52

• In peas, tall plants (T) are dominant to short
  plants (t). Round seeds (R) are dominant to
  wrinkled seeds (r). Show a cross between a plant
  with the genotype TtRr and a plant with the
  genotype TTRR.
• Possible gametes from TtRr TR, Tr, tR, tr
• Possible gametes from TTRR TR, TR, TR, TR

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TR
Tr
tR
tr
A.




TtRr
TTRR
TTRr
TrRR
TR
TR
TR
TR
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• What fraction of the offspring will be TtRr?
  
• What fraction of the offspring will be TTRR?
  
• What percent of the offspring will be tall with
  round seeds?

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• What fraction of the offspring will be TtRr?
  25
• What fraction of the offspring will be TTRR?
  
• What percent of the offspring will be tall with
  round seeds?

56

• What fraction of the offspring will be TtRr?
  25
• What fraction of the offspring will be TTRR?
  25
• What percent of the offspring will be tall with
  round seeds?

57

• What fraction of the offspring will be TtRr?
  25
• What fraction of the offspring will be TTRR?
  25
• What percent of the offspring will be tall with
  round seeds? 100

58
Dihybrid practice problems
59

• In guinea pigs, short hair (H) is dominant to
  long hair (h). Black eyes (B) are dominant to red
  eyes (b).

60

• In guinea pigs, short hair (H) is dominant to
  long hair (h). Black eyes (B) are dominant to red
  eyes (b).
• Two guinea pigs that are both heterozygous for
  hair length and eye color are mated.

61

• In guinea pigs, short hair (H) is dominant to
  long hair (h). Black eyes (B) are dominant to red
  eyes (b).
• Two guinea pigs that are both heterozygous for
  hair length and eye color are mated.
• What are the genotypes of the parents?

62

• In guinea pigs, short hair (H) is dominant to
  long hair (h). Black eyes (B) are dominant to red
  eyes (b).
• Two guinea pigs that are both heterozygous for
  hair length and eye color are mated.
• What are the genotypes of the parents? HhBb

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HB
Hb
hB
hb
B.




HhBb
FFDD
HHBb
HhBB
HB
HhBb
HHBb
HHbb
Hhbb
Hb
hB
HhBB
HhBb
hhBB
hhBb
hb
HhBb
Hhbb
hhbb
hhBb
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1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype.
   

65

1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype. 9331 ? Short
   hair black eyes Long hair black eyes Short hair
   red eyes long hair red eyes

66

1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype. 9331 ? Short
   hair black eyes Long hair black eyes Short hair
   red eyes long hair red eyes
2. What fraction of the offspring have the genotype
   HHbb?

67

1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype. 9331 ? Short
   hair black eyes Long hair black eyes Short hair
   red eyes long hair red eyes
2. What fraction of the offspring have the genotype
   HHbb? 1/16

68

1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype. 9331 ? Short
   hair black eyes Long hair black eyes Short hair
   red eyes long hair red eyes
2. What fraction of the offspring have the genotype
   HHbb? 1/16
3. What fraction of the offspring have the genotype
   HhBb?

69

1. What is the expected phenotypic ratio of the
   offspring? Be sure to indicate which number
   corresponds with which phenotype. 9331 ? Short
   hair black eyes Long hair black eyes Short hair
   red eyes long hair red eyes
2. What fraction of the offspring have the genotype
   HHbb? 1/16
3. What fraction of the offspring have the genotype
   HhBb? ¼

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• 2. A guinea pig that is heterozygous for both
  traits is crossed with a female that has long
  hair and red eyes.

71

• 2. A guinea pig that is heterozygous for both
  traits is crossed with a female that has long
  hair and red eyes.
• What is the genotype of the female parent?

72

• 2. A guinea pig that is heterozygous for both
  traits is crossed with a female that has long
  hair and red eyes.
• What is the genotype of the female parent? hhbb

73
HB
Hb
hB
hb
B.




hhbb
HhBb
Hhbb
hhBb
hb
hb
hb
hb
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1. What fraction of the offspring will have the
   genotype HhBb?

75

1. What fraction of the offspring will have the
   genotype HhBb? 1/4

76

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb?

77

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb? 1/4

78

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb? 1/4
3. What fraction of the offspring will have short
   hair and red eyes?

79

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb? 1/4
3. What fraction of the offspring will have short
   hair and red eyes? 1/4

80

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb? 1/4
3. What fraction of the offspring will have short
   hair and red eyes? 1/4
4. What fraction of offspring will have long hair?
   

81

1. What fraction of the offspring will have the
   genotype HhBb? 1/4
2. What fraction of the offspring will have the
   genotype hhbb? 1/4
3. What fraction of the offspring will have short
   hair and red eyes? 1/4
4. What fraction of offspring will have long hair?
   1/2