epistasis
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EpistasisTRANSCRIPT
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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 AaBb X AaBb Where A is dominant to a and B is dominant to b
If these two genes govern two different traits 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 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
Epistatic Gene Interactions
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A Cross Producing a 9:7 ratio
Figure 4.18
9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp
purple white
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Epistatic Gene Interaction
Complementary gene action Enzyme C and enzyme P cooperate to
make a product, therefore they complement one another
Enzyme C Enzyme P
Purple pigment
Colorless intermediate
Colorless precursor
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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
Colorless intermediate
Purple pigment
Enzyme C Enzyme P
Epistatic Gene Interaction
genotype cc
genotype pp
Colorless precursor
Colorless intermediate
Purple pigment
Enzyme C Enzyme P
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Inheritance of the Cream-Eye allele in Drosophila a rare fly with cream-colored eyes identified in a
true-breeding culture of flies with eosin eyes possible explanations
1. Mutation of the eosin allele into a cream allele 2. Mutation of a 2nd gene that modifies expression of the
eosin allele
Epistasis of Involving Sex-linked Genes
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The Hypothesis
Cream-colored eyes in fruit flies are due to the effect of a second gene that modifies the expression of the eosin allele
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Figure 4.19
Testing the Hypothesis
cream allele is recessive to +
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Interpreting the Data
Cross OutcomeP 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
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Interpreting the Data
Cross OutcomeP 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 –
151 + eye: 44 we eye: 14 ca eyea 12 : 3 : 1 ratio
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Modeling the Data
Cream phenotype is recessive therefore the cream allele is recessive allele (either sex-linked or autosomal)
The mutated allele of the cream gene modifies the we allele, while the wt cream allele does not C = Normal allele
Does not modify the eosin phenotype ca = Cream allele
Modifies the eosin color to cream, does not effect wt or white allele of white gene.
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Male gametes
CY
CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+YCXw+
CXw+ caXw+ caY
CXw-e
caXw+
caXw-e
CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY
CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y
CcaXw+Xw-e CcaXw-eY cacaXw+Xw-e cacaXw-eY
Fem
ale
gam
etes
Putative genotypes in a crossP w+/ w+; C/C x we/Y; ca/ca
F1 w+/ we; C/ca & w+/Y; C/ca
F2 ¾ C/_ x ¾ w+/_ ¼ we/Y
¼ ca/ca x ¾ w+/_ ¼ we/Y
9/16 C/_ ; +3/16 ca/ca; +3/16 C/_ ; we
1/16 ca/ca; we
Modeling the Data
red
eosincream
12:3:1
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Inheritance of comb morphology in chicken First example of gene interaction William Bateson and Reginald Punnett in 1906 Four different comb morphologies
A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio
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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 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
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Gene Interaction
Duplicate gene action Enzyme 1 and enzyme 2
are redundant They both make product
C, therefore they duplicate each other
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Duplicate Gene Action Epistasis
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
TTVVTriangular
ttvvOvate
TtVvAll triangular
F1 (TtVv) x F1 (TtVv)
x
F1 generation
15:1 ratio results
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Bombay Phenotype
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Bombay Phenotype
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Bombay Phenotype
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Categories of Inheritance Paterns
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Com
plem
enta
ry a
ctio
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Dup
lica
te a
ctio
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Epi
stas
is o
f aa
ove
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-
Epi
stas
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f A
- ov
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b
Generation of Epistatic Ratios
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