GENETICS (EPISTASIS)

1. knip

2. Epistasis
and
Modifier Genes
Pseudoalleles

3. EPISTASIS
Definition: Suppression of the effect of a gene by another
gene.
mask
A B
Epistatic/Dominating
Gene
(the gene masking the effect of
another gene)
Regulated/Hypostatic
Gene
(gene whose functionality of the
expression is masked or prevented)
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4. Epistatic Gene
A bBa
Epistatic
Gene
Hypostatic
Gene
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5. Types of Epistasis
A
Dominant
Epistasis
a
Recessive
Epistasis
Epistatic
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6. Dominant Epistasis
A bBa
Epistatic
Gene
Hypostatic
Gene
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7. Recessive Epistasis
a bBa
Epistatic
Gene
Hypostatic
Gene
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8. Mendelian Dihybrid Phenotypic Ratio
After the cross in F2
9 : 3 : 3 : 1
9 A_B_
3 A_bb
3 aaB_
1 aabb
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9. Dominant Epistasis
Example:
Color of summer squash = 2 seperate genes
A/a - White - Epistatic gene
B - Yellow - hypostatic gene
b - Green
A B AA
Aa
aa
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10. Dominant Epistasis
AaBb x AaBb
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
white
white
white whitewhite
white whitewhite
whitewhite
white white greenyellow
yellowyellow
Ratio: 12:3:1
12-white
3-yellow
1-green
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11. Recessive Epistasis
Example: Color of mice = 2 seperate genes
C - pigment
c - no pigment - Epistatic Gene
B - more pigment (black)
b - less pigment (brown)
cc - albino
c B
CC
Cc
cc
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12. Recessive Epistasis
Genotype Coat Color
9 B_C_ Black
3 B_cc Albino
3 bbC_ Brown
1 bbcc albino
Ratio: 9:3:4
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13. Duplicate Dominant
Example:Kernel Color in Wheat
Product
Enzyme B
Gene B
Precursor
Enzyme A
Gene A
Genotype Kernel Phenotype Enzymatic Activities
9 A_B_ colored kernels functional enzyme
from both genes
3 A_bb colored kernels functional enzyme
from the A gene pair
3 aaB_ colored kernels functional enzyme
from the B gene pair
1 aabb colorless kernels non-functional
enzymes produced at
both genesRatio: 15:1
BA OR
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14. Duplicate Recessive:Complementary Genes
Example:Flower color in sweet pea
Intemediate
Enzyme B
Gene P
Precursor
Enzyme C
Gene C
Genotype Kernel
Phenotype
Enzymatic
Activities
9 C_P_ Flowers colored,
anthocyonin
produced
functional enzyme
from both genes
3 C_pp Flowers white,
no anthocyonin
produced
p enzyme non-
functional
3 ccP_ Flowers white,
no anthocyonin
produced
c enzyme non-
functional
1 ccpp Flowers white,
no anthocyonin
produced
c and p enzyme
non-functional
Ratio: 9:7
Anthocyonin
C PAND
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15. Dominant Recessive
Genotype Phenotype and genetic explanation
9 K_D_ no malvidin because dominant D allele is present
3 K_dd maldivin productions because dominant D allele is
absent
3 kkD_ no malvidin because recessive k and dominant D
alleles are present
1 kkdd no malvidin because recessive k allele is present
D
K malvidin
production
regulate the K gene
Ratio: 13:3
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Example: Production of
Malvidin in the plant of
Primula

16. TYPE OF EPISTASIS RATIO
Dominant 12:3:1
Recessive 9:3:4
Duplicate Dominant 15:1
Duplicate Recessive 9:7
Dominant Recessive 13:3
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17. Modifier Genes
A gene that can modify the effects or
expression of another gene.
Example : coat color of the mice
Gene - controls the color
B - black
b - brown
Gene - influence the color
D - fullcolor
d - dilute or faded color
Genotype Coat Color
9 B_D_ black
3 B_dd dilute black
3 bbD_ brown
1 bbdd dilute brown
The D gene does not mask the effect of B gene, rather it modifies its
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18. Pseudoalleles
State in which two genes with similar functions are located
so close to one another on a chromosome that they are
genetically linked.
Characeristics of Pseudoalleles
1. These are closely linked allele within which crossing over
occur.
2. They affect the same character.
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19. Example of Pseudoallele
Red eye color Drosophila
has different mutants like
white and apricot. They
affect pigmentation i.e.,
affect the same character.
So they are allelic. They can
undergo recombination.
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