Download Brooker Chapter 4

Epistatic Gene Interactions
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Gene interactions occur when two or more different
genes influence the outcome of a single trait
Most morphological traits (height, weight, color) are
affected by multiple genes
Epistasis describes situation between various alleles
of two genes
Quantitative loci is a term to describe those loci
controlling quantitatively measurable traits
Pleiotropy describes situations where one gene affects
multiple traits
Epistatic Gene Interactions
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examine cases involving 2 loci (genes) that each
have 2 alleles
Crosses performed can be illustrated in general by
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AaBb X AaBb
Where A is dominant to a and B is dominant to b
If these two genes govern two different traits
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A 9:3:3:1 ratio is predicted among the offspring
simple Mendelian dihybrid inheritance pattern
If these two genes do affect the same trait the 9:3:3:1
ratio may be altered
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9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1
epistatic ratios
A Cross Producing a 9:7 ratio
Figure 4.18
9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp
purple
white
Epistatic Gene Interaction
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Complementary gene action
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Enzyme C and enzyme P cooperate to
make a product, therefore they
complement one another
Enzyme C
Colorless
precursor
Enzyme P
Colorless
intermediate
Purple
pigment
Epistatic Gene Interaction
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Epistasis describes the situation in which a gene masks the
phenotypic effects of another gene
Epistatic interactions arise because the two genes encode
proteins that participate in sequence in a biochemical
pathway
If either loci is homozygous for a null mutation, none of that
enzyme will be made and the pathway is blocked
Colorless
precursor
Enzyme C
Colorless
intermediate
Enzyme P
Purple
pigment
genotype cc
Colorless
precursor
Enzyme C
Colorless
intermediate
Enzyme P
genotype pp
Purple
pigment
Epistasis of Involving Sex-linked Genes
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Inheritance of the Cream-Eye allele in
Drosophila
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a rare fly with cream-colored eyes identified in a
true-breeding culture of flies with eosin eyes
possible explanations
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1. Mutation of the eosin allele into a cream allele
2. Mutation of a 2nd gene that modifies expression of the
eosin allele
The Hypothesis
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Cream-colored eyes in fruit flies are due to the
effect of a second gene that modifies the
expression of the eosin allele
Testing the Hypothesis
cream allele is recessive
to +
Figure 4.19
Interpreting the Data
Cross
Outcome
P cross:
Cream-eyed male X
wild-type female
F1: all red eyes
F1 cross:
F1 brother X F1 sister
F2: 104 females with red eyes
47 males with red eyes
44 males with eosin eyes
14 males with cream eyes
F2 generation contains males with eosin eyes
This indicates that the cream allele is
not in the same gene as the eosin allele
Interpreting the Data
Cross
Outcome
P cross:
Cream-eyed male X
wild-type female
F1: all red eyes
F1 cross:
F1 brother X F1 sister
F2 generation contains –
151 + eye: 44 we eye: 14 ca eye
a 12 : 3 : 1 ratio
F2: 104 females with red eyes
47 males with red eyes
44 males with eosin eyes
14 males with cream eyes
Modeling the Data
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Cream phenotype is recessive therefore the
cream allele is recessive allele (either sexlinked or autosomal)
The mutated allele of the cream gene modifies
the we allele, while the wt cream allele does
not
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C = Normal allele
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Does not modify the eosin phenotype
ca = Cream allele
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Modifies the eosin color to cream, does not effect wt or
white allele of white gene.
Modeling the Data
Putative genotypes in a cross
P w+/ w+; C/C x we/Y; ca/ca
F1 w+/ we; C/ca & w+/Y; C/ca
Male gametes
F2 ¾ C/_ x ¾ w+/_
¼ we/Y
9/16
3/16
3/16
1/16
x
¾ w+/_
¼ we/Y
C/_ ; +
red
ca/ca; +
C/_ ; we eosin
ca/ca; we cream
12:3:1
CXw+
Female gametes
¼ ca/ca
CXw+
CY
caXw+
caY
CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+Y
CXw-e CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY
caXw+ CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y
caXw-e CcaXw+Xw-e CcaXw-eY cacaXw+Xw-e cacaXw-eY
A Cross Involving a Two-Gene Interaction Can
Still Produce a 9:3:3:1 ratio
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Inheritance of comb morphology in chicken
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First example of gene interaction
William Bateson and Reginald Punnett in 1906
Four different comb morphologies
Figure 4.17b
The crosses of Bateson and Punnett
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F2 generation consisted of chickens with four
types of combs
 9 walnut : 3 rose : 3 pea : 1 single
Bateson and Punnett reasoned that comb
morphology is determined by two different
genes
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R (rose comb) is dominant to r
P (pea comb) is dominant to p
R and P are codominant (walnut comb)
rrpp produces single comb
Gene Interaction
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Duplicate gene action
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Enzyme 1 and enzyme 2 are
redundant
They both make product C,
therefore they duplicate
each other
Duplicate Gene Action
Epistasis
x
TTVV
Triangular
ttvv
Ovate
F1 generation
TtVv
All triangular
F1 (TtVv) x F1 (TtVv)
15:1 ratio results
TV
Tv
tV
tv
TV
Tv
tV
tv
TTVV
TTVv
TtVV
TtVv
TTVv
TTvv
TtVv
Ttvv
TtVV
TtVv
ttVV
ttVv
TtVv
Ttvv
ttVv
ttvv
(b) The crosses of Shull
Bombay Phenotype
Bombay Phenotype
Bombay Phenotype
Categories of Inheritance Paterns
Duplicate action
Complementary action
Epistasis of A- over bb
Epistasis of aa over B-
Generation of Epistatic Ratios