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8/10/2019 7. Inter-Allelic Interactions
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G E N E T I C S-1
MODULE 5
EXTENSION OF MENDELIAN INHERITANCE-2
INTER-ALLELIC INTERACTIONS
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Describe that inter-allelic interactions stem from the
relationship between alleles of different genes.
Describe the different classes of epistasis (complementary,
inhibitory and duplicate) and the types of epistasis within each
of the classes.
Outline and explain the various inter-allelic relationships and
the underlying biochemical basis for inhibitory and duplicated
epistases, clearly showing in each case how they modify
expected Mendelian phenotypic ratios.
LEARNING OUTCOMES
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INTERALLELIC INTERACTIONS (Epistasis)
Mendelian dihybrid ratios of 9:3:3:1 in the F2and 1:1:1:1 ratio in the
test cross are altered by epistatic interactions.
Inter-allelic interactions or epistasisoccur when two or more
genes specify enzymes which catalyze steps in a common
biochemical pathway.
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In epistasis, the interacting genes produce enzymes that function
in the same biochemical pathway.
Each enzyme acts at a different step in the biochemical pathway.
PRECURSOR END PRODUCTINTERMEDIATE
Gene 1 Gene 2
Enzyme 1 Enzyme 2
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INTERALLELIC INTERACTIONS (Epistasis)
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INTER-ALLELIC INTERACTIONS
The interacting genes may have an apparent dominant/recessive
type relationship.
The alleles at one gene locus appear to suppress the alleles at
another locus that affects the same phenotype.
A gene that suppresses another is referred to as being epistatic.
The gene that is suppressed is referred to as being hypostatic.
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Epistasis can fall into one of THREEmain categories:
1. Complementary epistasis
Two genes complement each other to produce one finalphenotype.
2. Inhibitory epistasis
An inhibitor or repressor is involved.
3. Duplicate epistasis
Two duplicate pathways lead to the same end product.
G E N E T I C S-I
INTERALLELIC INTERACTIONS (Epistasis)
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Assume two independent genes with complete dominance
governs coat colour in mice.
One of the two genes in the homozygous recessive form is
epistatic over the other.
Lets say the epistatic gene is A
and the hypostatic gene is B.
Then aa (homozygous recessive state) is epistaticover B
regardless of the form it is in (BB, Bb, bb).
1. RECESSIVE EPISTASIS
Complementary epistasis
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Recessive epistasis: Coat colour in mice
AGOUTI BLACK ALBINO
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RECESSIVE EPISTASIS
Recessive epistasis
Agouti Albino
AABB x aabb
AaBb (Agouti) self
F2Genetic Explanation
9 A-B- = Agouti 9
3 A-bb = Black 3
3 aaB- =
1 aabb =Albino 4
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Test cross for Recessive Epistasis
Recessive epistasis
Agouti Albino
AABB x aabb
AaBb (Agouti) F1
Test cross: AaBb (Agouti) x aabb (albino, double recessive)
Testcross Genetic Explanation
1 AaBb = Agouti 1
1 Aabb = Black 1
1 aaBb =
1 aabb = Albino 2
AB Ab aB ab
ab AaBb Aabb aaBb aabb
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Recessive Epistasis: A Biochemical Basis
PRECURSOR(Albino) END PRODUCT(Agouti)INTERMEDIATE(Black)
GeneA- Gene B-
EnzymeA EnzymeB
NOTE: Both genes A and B complement each
other to generate the final product (agouti).
COMPLEMENTARY EPISTASIS
9 A-B- = 9 Agouti
3 A-bb = 3 Black
3 aaB- =
1 aabb =4 Albino
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G E N E T I C SRECESSIVE EPISTASIS
Regulatory gene - produces a
regulatory protein (or activator),
which is required to transcriptionallyactivate a structural gene.
Recessive epistasis occurs:
two gene loci produce enzymes
which are part of the same
biochemical pathway (or)
when one gene transcriptionally
activates or regulates another gene
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Assume TWO independent genes with complete dominance control
the production of hydrocyanic acid in clover.
Definition of duplicate recessive epistasis: Either one of the twogenes in the homozygous recessive state is epistatic over the other
gene, i.e.:
Gene Ain its recessive form aa is epistatic over gene Bregardlessof its form (BB, Bb or bb)
and
Gene Bin its recessive form bb is epistatic over gene Aregardless
of its form (AA, Aa or aa).
2. DUPLICATE RECESSIVE EPISTASIS
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Hydrocyanic Acid (HCN)
production in clover
P: AABB aabb
High [HCN] no [HCN]
F1: AaBb
High [HCN]
Self
F2: A-B- 9 High [HCN]
A-bb 3
aaB- 3 No [HCN]
aabb 1
Modified F2ratio- 9:7
Test-cross: AaBb x aabb
AaBb 1 High [HCN]
Aabb 1
aaBb 1 No [HCN]
aabb 1
Modified Test-cross ratio- 1:3
X
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Duplicate Recessive Epistasis: A Biochemical Basis
PRECURSOR(No HCN) END PRODUCT(HCN)INTERMEDIATE(Cyanic glucoside)
GeneA- Gene B-
EnzymeA EnzymeB
NOTE: Both genes A and B complement
each other to generate the final product
(high [HCN])
COMPLEMENTARY EPISTASIS
9 A-B- = 9 HCN
3 A-bb =
3 aaB- =
1 aabb =7 [No HCN]
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Assume two independent genes with complete dominance.
One gene in the dominant state is epistatic over the other gene locusregardless of the form it is in (homozygous dominant, heterozygous or
homozygous recessive).
e.g. Consider genes Aand B that control a single phenotype.
If the gene Alocus is epistatic and the gene Blocus is hypostatic, thenAAand Aaare epistaticover either BB, Bb or bb.
3. DOMINANT EPISTASISINHIBITORY EPISTASIS
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Fruit colour inSummer SquashP: AABB aabb
White Green
F1: AaBb
White
F2:A-B- 9 White
A-bb 3
aaB- 3 Yellow
aabb 1 Green
Modified F2ratio- 12:3:1
Test-cross: AaBb x aabbAaBb 1 White
Aabb 1
aaBb 1 Yellow
aabb 1 Green
Modified Test-cross ratio- 2:1:1
X
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DOMINANT EPISTASIS
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Dominant Epistasis: A Biochemical Basis
PRECURSOR 1
(White)
END PRODUCT
(Yellow)
INTERMEDIATE
(Green)
GeneA-
Gene B-
EnzymeA
EnzymeB
NOTE: Gene A inhibits the production of the
final product (yellow squash colour).
INHIBITORY EPISTASIS
PRECURSOR 2 INHIBITOR
9 A-B- =
3 A-bb =
3 aaB- = 3 Yellow
1 aabb = 1 Green
12 white
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Assume two independent genes with complete dominance.
One gene in the dominant state is epistatic over the other gene locus
regardless of the form it is inAND
The other gene in its homozygous recessive form is epistatic over the
previous gene locus regardless of the form it is in.
e.g. Consider genes Aand Bthat control a single phenotype
Then AAand Aaare epistaticover either BB, Bb or bb
And bbis epistaticover either AA, Aa or aa.
4. DOMINANT AND RECESSIVE INTERACTION
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Flower colour
(malvidin) in
Primula
P: AABB aabb
White White
F1: AaBb
White
F2:
A-B- 9 White
A-bb 3
aaB- 3 Blue
aabb 1 White
Modified F2ratio- 13:3
Test-cross: AaBb x aabb
AaBb 1 White
Aabb 1
aaBb 1 Blue
aabb 1 White
Modified Test-cross ratio- 3:1
X
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4. DOMINANT AND RECESSIVE INTERACTION
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Dominant and Recessive Interaction: A Biochemical Basis
PRECURSOR 1
(White)
END PRODUCT
(Blue)
INTERMEDIATE
(White)
GeneA-
Gene B-
EnzymeA
EnzymeB
NOTE: Gene A inhibits the production of
the final product (blue flower colour).
INHIBITORY EPISTASIS
PRECURSOR 2 INHIBITOR
9 A-B- =
3 A-bb =
3 aaB- = 3 Blue
1 aabb = 1 white
12 white
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Assume two independent genes with complete dominance
Either of the two genes in the dominant state is epistatic over the other
gene locus regardless of the form it is in.
e.g. Consider genes Aand Bthat control a single phenotype
Then AAandAaare both epistaticover either BB, Bb or bb
And both BBandBbare epistaticover either AA, Aa or aa.
5. DUPLICATE DOMINANT EPISTASIS
Duplicated Epistasis
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Feathered
Chicken Shank
P: FFSS ffss
Feathered Shank Clean Shank
F1: FfSs
Feathered Shank
F2:
F-S- 9
F-ss 3
ffS- 3
ffss 1 Clean
Modified F2ratio- 15:1
Test cross: FfSs x ffss
Ff Ss 1
Ff ss 1
ff Ss 1
ff ss 1 Clean
Modified Test-cross ratio- 3:1
X
Feathered Feathered
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Duplicated Epistasis
http://www.google.tt/url?sa=i&rct=j&q=feathered+shank&source=images&cd=&cad=rja&docid=5GKxbIx7EZ7J6M&tbnid=WqbG8l9G0hIQAM:&ved=0CAUQjRw&url=http://www.backyardchickens.com/t/299038/icelandic-chickens/16750&ei=yFwRUZztOuW90QGewYHYBA&bvm=bv.41867550,d.dmQ&psig=AFQjCNHPy78sDYNbspB3k4SoYnmTgtWdiQ&ust=13601787168113768/10/2019 7. Inter-Allelic Interactions
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6. Duplicate DOMINANT EPISTASIS: A Biochemical Basis
PRECURSOR 1
(Clean)
END PRODUCT
(Feathered)
Gene S-
Gene F-
EnzymeS
EnzymeF
NOTE: Gene F and S produce the same end
product (feathered chicken shank).
DUPLICATED EPISTASIS
PRECURSOR 2(Clean) 9 F-S- =
3 F-ss =
3 ffS- =
1 ffss = 1 Clean shank
15 Feathered
shank
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Each of the two genes in the dominant state (homozygous dominant
or heterozygous) produces the same phenotype. When both genes arepresent in the dominant state a novel phenotype is generated.
e.g.
Consider genesA and Bthat control a single phenotype
Then AA /AaORBB/Bbwill produce the same phenotype.
When both AA /AaANDBB /Bbare present, a novel phenotype is
generated.
7. DUPLICATE INTERACTION
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Fruit shape
in squashP: AABB aabb
Disc Pyriform (long)
F1: AaBb
Disc
F2
:
A-B- 9 Disc
A-bb 3
aaB- 3
aabb 1 Pyriform
Modified F2ratio- 9:6:1
Test-cross: AaBb x aabb
AaBb 1 Disc
Aabb 1
aaBb 1
aabb 1 Pyriform
Modified Test-cross ratio- 1:2:1
X
SphericalSpherical
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7. DUPLICATE INTERACTION
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Duplicate Interaction: A Biochemical Basis
PRECURSOR 1
(Pyriform)
Spherical
Spherical
GeneA-
Gene B-
EnzymeA
EnzymeB
NOTE: Gene A and B produce the same end
product (spherical shape).
PRECURSOR 2(Pyriform)
Disc
9 A-B- = 9 Disc
3 A-bb =
3 aaBb =
1 aabb = 1 pyriform
6 Spherical
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Epistasis is interactions between alleles of different genes.
There are three types of epistasis
Complementary epistasistwo genes complement each other Inhibitory epistasisan inhibitor or a repressor is involved
Duplicate epistasistwo duplicate pathways lead to same product.
Epistasis ONLYmodify Mendelian phenotypic ratios.
Epistasis causes many different genotypes to fall into one phenotypic
category. (Just like dominance)
SUMMARY
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