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White
CCpp
P
Cp
Purple
CcPp
F1
X
White
ccPP
X
Cp
Purple
CcPp
F2
CP
Cp
cP
CP
CCPP
Purple
CCPp
Purple
CcPP
Purple
CcPp
Purple
Cp
CCPp
Purple
CCpp
White
CcPp
Purple
Ccpp
White
cP
CcPP
Purple
CcPp
Purple
ccPP
White
ccPp
White
cp
CcPp
Purple
Ccpp
White
ccPp
White
ccpp
White
Summary: 9/16 purple, 7/16 white
cp
AABB
X
aabb
ab
AB
AaBb
AB
P
X
Ab
Gametes
AaBb
aB
F1 x F1
ab
AB
White
White
White
White
Ab
White
White
White
White
aB
White
White
Yellow
Yellow
ab
White
White
Yellow
Green
Where A--- is white, aaB- is yellow, and aabb is green.
aabb
aaBB
aaBb
AAbb
Aabb
AABB
AABb
AaBB
AaBb
Genotypes
Interactions
_______________________________________________________________________________________
Epistasis
Dominance
Phenotypic Classes
Example
Phenotypes
_______________________________________________________________________________________
(Classical Complete
ratio)
for A,B
9
3
3
1
Garden peas
seed color
and surface
texture
9 yellow round
3 yellow
wrinkled
3 green round
1 green
wrinkled
None
Complete
X-X-
3
0?
1
Guinea pig
3 short yellow
for A,
incomplete
for B
-X-
hair length
and color
(X)
6 short cream
(-)
3 short white
1 long yellow
(0)
2 long cream
(? )
1 long white
?
66
aabb
aaBB
aaBb
AAbb
Aabb
AABB
AABb
AaBB
AaBb
_______________________________________________________________________________________
Epistasis
Dominance
Phenotypic Classes
Example
Phenotypes
_______________________________________________________________________________________
aa epistatic
Complete
9 3
4
Mice coat pattern
9 agouti
to B,b
for A,B
and color or
3 colored
(Recessive)
Labrador
4 white
retriever color
A epistatic
to B,b
(Dominant)
Complete
for A,B
12
3
1
Summer squash
fruit color or
Foxglove color
12 white
3 yellow
1 green
A epistatic
to B,b; bb
epistatic to
A,a (Dominant
and Recessive)
Complete
for A,B
13
3
1
Chicken color
13 white
3 colored
aa epistatic
to B,b; bb
epistatic to
A,a (Duplicate
Recessive)
Complete
for A,B
9
Daisy color
or white and
purple flower
color
9 purple
7 yellow
center
A epistatic
to B,b; B
epistatic to
A,a (Duplicate
Dominant)
Complete
for A,B
15
Shepherd's
purse seed
capsules
15 triangular
1 ovoid
7
1
66 cont.
Epistatic gene interaction:
1) One distinct phenotype in each phenotypic classrather than two, although two separate genes are
being considered.
2) “Novel” or new phenotypes may appear in the F1
and/or F2 generations that had not appeared in the P
generation.
3) Phenotypic ratio in F2 is altered from classical ratio
(ex. 9:3:4, 12:3:1, 13:3, 9:7, 15:1)
4) “Masking” effect. Masking (non-allelic) is not the
same as dominance (allelic).
Five forms of epistasis and their
corresponding F2 phenotypic ratios:
• 1) Recessive (9:3:4), aa epistatic to B,b.
• 2) Dominant (12:3:1), A epistatic to B,b.
• 3) Dominant and Recessive (13:3), A epistatic to
B,b, and/or bb epistatic to A,a.
• 4) Duplicate Recessive (9:7), aa epistatic to B,b,
and/or bb epistatic to A,a.
• 5) Duplicate Dominant (15:1), A epistatic to B,b,
and/or B epistatic to A,a.
9 A _B_ : 3 A_bb : 3 aaB_ : 1 aabb
• 1) 9 A _B_ : 3 A_bb : 4 aaB_ , aabb
• 2) 12 A _B_ , A_bb : 3 aaB_ : 1 aabb
• 3) 13 A _B_ , A_bb , aabb: 3 aaB_
• 4) 9 A _B_ : 7 A_bb , aaB_ , aabb
• 5) 15 A _B_ , A_bb , aaB_ : 1 aabb
Epistatic Metabolic models:
Step - Type
1) Recessive
Allele A
Allele B
Enzyme A
White
(Precursor)
Enzyme B
Colored
(Intermediate)
Agouti
(Final Product)
2) Duplicate Recessive
Allele A
Allele B
Enzyme A
Enzyme B
Yellow
(Precursor)
Yellow
(Intermediate)
Purple
(Final Product)
Simultaneous
1) Duplicate Dominant
Allele A
Enzyme A
Triangular
(Final Product)
Ovoid
(Precursor)
Enzyme B
Allele B
Metabolic pathway models at which 3 of
the 5 forms operate:
• 1) Step-type:
• Recessive (9:3:4)-2 conversions and 3 metabolites
(phenotypes)-precursor, intermediate, and final product are
different.
• Duplicate Recessive (9:7)-2 conversions and 3 metabolites
(phenotypes)-precursor and intermediate are the same, but
final product different.
• 2) Simultaneous:
• Duplicate Dominant (15:1)-1 conversion and 2 metabolites
(phenotypes)-precursor and final product are different,
with no intermediate.
Illustrate recessive epistasis by filling in all blanks below. Circle all gametes. aa epistatic to B,b.
A_B_ = agouti mouse, A_bb = colored mouse; aaB_, aabb = white mouse.
P
Phenotypes
Genotypes
Gametes
F1 Phenotypes
Genotypes
Gametes
F2 Genotypes
F2 Phenotypes
F2 Phenotypic ratio
F2 Genotypic ratio
x
x
x
x
x
x
AAbb
.
aaBB
..
..
..
..
.
. .
. .
..:
..:
USE THE INFORMATION ABOVE TO MAKE A TESTCROSS BELOW
F1 Parent
Testcross Parent
Phenotypes
x
Genotypes
x
Gametes
x
Testcross Progeny
Phenotypes
Genotypes
.
.
.
.
.
.
Testcross Phenotypic Ratio
Testcross Genotypic Ratio
.
.
Illustrate recessive epistasis by filling in all blanks below. Circle all gametes.
aa epistatic to B,b. A_B_ = agouti mouse, A_bb = colored mouse;
aaB_, aabb = white mouse.
P
F1
Phenotypes colored
x
white
Genotypes
AAbb
x
aaBB
Gametes
1/1 Ab
x
1/1 aB
Phenotypes agouti
x
agouti
Genotypes
AaBb
x
AaBb
Gametes 1/4 AB 1/4 Ab 1/4 aB 1/4 ab x 1/4 AB 1/4 Ab 1/4 aB 1/4 ab
F2 Genotypes
F2 Phenotypes
F2 Phenotypic ratio
F2 Genotypic ratio
.
.
.
.
Working space
Working space
9:3:4
4:2:2:2:2:1:1:1:1
.
.
.:
.:
.
USE THE INFORMATION ABOVE TO MAKE A TESTCROSS BELOW
F1 Parent
Testcross Parent
Phenotypes agouti
x
Genotypes AaBb
x
Gametes 1/4 AB 1/4 Ab 1/4 aB 1/4 ab x
white
aabb
1/1 ab
Testcross Progeny
Phenotypes
agouti
colored
.
Genotypes
white
white
Testcross Phenotypic Ratio
Testcross Genotypic Ratio
1/4 AaBb
1/4 Aabb
2:1:1
1:1:1:1
63
1/4aaBb
1/4 aabb
.
.
.
.
.
Example of recessive epistasis
(Labrador retriever coat color):
A = allows for deposition of either black or brown color in coat;
aa = prevents deposition of color (black or brown) pigment in coat
(epistatic gene pair);
B = black; b = brown.
Photo (a)
A B
Photo (c)
*aaB
A bb
Photo (b)
*This dog has a black nose and black lips and would, therefore, be aaB_ (lacks
black pigment in hair, but not in skin). If it was aabb it would have yellow hair
and light colored skin [see nose and lips of dog in photo (b)].