Download Gene Interaction and Epistasis

Gene Interaction and Epistasis
• In each of these problems you examine a single phenotypic trait that is determined by 2 genes. Each of the genes has a
dominant allele and a recessive allele.
• In each problem you start with a set of three true-breeding strains. For
example, three plants with either red, purple or white flowers.
• In these problems two main goals are to: - determine the genotype of each of the 3 true-breeding strains;
th
- determine the genotype and phenotype of the 4 true-breeding strain.
• You will determine the genotype of each of the true-breeding strains by (a) crossing pairs of the 3 given strains, then (b)
performing an intercross on the first generation (F1) offspring in each cross, for example:
F1
F2
ratio
all
ratio
3
1
Cross 1
red x purple
red
F1 Intercross
red
purple
_____________________________________________________________________________________________________
Review: Traits Controlled by One Gene
In the simplest case genetic traits are controlled by a single gene with two alleles and simple dominance. The traits Mendel
studied in peas are like this. For example, pea color was controlled by a dominant yellow allele and a recessive green allele.
In this situation there are four possible genotypes:
4 Total Genotypes
Genotype
GG
Gg
gG
gg
There are only two functional genotype categories,
since there are just two phenotypes:
2 Functional Genotypes
Phenotype
yellow
yellow
yellow
green
Genotype
Ggg
Phenotype
yellow
green
The three genotypes with at least one dominant allele are yellow all have yellow peas. We use the abbreviation “G-“ this
functional genotype category. This notation represents all the genotypes with at least one dominant G allele; the “-“
indicates it doesn’t matter if the second allele is dominant or recessive.
True-breeding genotypes. Two of these genotypes are true-breeding (homozygous): GG yellow and gg green. If we cross
these true-breeding strains:
Yellow (GG) x Green (gg)
st
all the first generation (F1) offspring will inherit a dominant allele from the 1 parent and a recessive allele from the other:
F1 100% Yellow (Gg)
If we perform an intercross on these first generation offspring:
Yellow (Gg) x Yellow (Gg)
¾ of the offspring will fall in the G- category, and ¼ will fall in the gg category, so we sill see yellow and green offspring in
the ratio 3 to 1:
Phenotype
Ratio
Genotype
__Yellow________3_________G-____
__ Green________1_________gg____
Traits Controlled By Two Genes
As Mendel showed, when there are two genes with two alleles of each, there are 16 possible genotypes. If each gene
controls a different phenotype, there are four different phenotype categories. For instance,
yellow peas and smooth skin
yellow peas and wrinkled skin
green peas and smooth skin
green peas and wrinkled skin
and the 16 genotypes fall into 4 functional genotype classes:
Genotypes
Ratio
Summary
Phenotype
At least one dominant
of both genes
AABB
AaBB
aABB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
A-Byellow, smooth
A least one dominant
allele of A, but not B
AAbb
Aabb
aAbb
A least one dominant
allele of B, but not A
aaBB
aaBb
aabB
No dominant alleles of
either gene
aabb
3
aaByellow, wrinkled
3
A-bb
green, smooth
1
aabb
green, wrinkled
In this situation there are four doubly homozygous, true-breeding genotypes:
True breeding
AABB
aaBB
AAbb
aabb
When two genes control a single phenotypic trait, we have the same four functional genotype classes, and can have as
many as four phenotypes:
Two Genes and Two Traits:
Genotype
A-BaaBA-bb
aabb
Phenotype___
yellow, smooth
yellow, wrinkled
green, smooth
green, wrinkled
Two Genes and One Trait:
Genotype
A-B__aaB__A-bb
__aabb
_
Phenotype___
red flowers
orange flowers
yellow flowers
white flowers
Epistasis. But different true-breeding genotypes can have the same phenotype, and the four true-breeding
strains may only have 3 different phenotypes or even 2 different phenotypes, as described below.
Crossing Strains That are True-Breeding for Two Genes
One Gene Segregating
If we cross these two true-breeding genotypes:
AABB x AAbb
Each offspring will inherit
a dominant A allele from both parents,
a dominant B allele from the first parent, and
a recessive b allele from the second parent:
AABb
In this cross, we say one gene segregated (B).
When we intercross these F1 offspring:
AABb x AABb
We will obtain four F2 offspring genotypes:
• All strains will have 2 dominant A alleles.
AABB
AABb
AAbB
AAbb
• Three strains will have at least one at least one
dominant B allele will have the same phenotype.
• The fourth strain with two recessive bb alleles will
generally have a different phenotype.
Since the four genotypes have equal frequencies,
there will be a 3-1 ratio in the F2 offspring frequencies
with the two phenotypes.
Two Genes Segregating
If we cross these two true-breeding genotypes:
AAbb x aaBB
Each offspring will inherit
a dominant A allele & recessive b allele from the first parent, and
a recessive a allele & dominant B allele from the second parent:
AaBb
In this cross, we say two genes segregated (both A and B).
When we intercross these F1 offspring:
AaBb x AaBb
This is like Mendel’s di-hybrid cross, and we will obtain 16 genotypes,
as Mendel did:
• 9 will have at least one dominant allele of both genes
AABB, AaBB, aABB, AABb, AaBb,
AabB, aABB, aABb, aAbB
• 3 will have at least one dominant A allele and recessive bb alleles
AAbb, Aabb, aAbb
• 3 will have recessive aa alleles and at least one dominant B allele
aaBB, aaBb, aabB
• 1 will have recessive aa alleles and recessive bb alleles
aabb
• If there is no epistasis, we will observe four phenotypes in the
familiar 9-3-3-1 ratios.
Pathways that result in Genetic Epistasis
Two genes can act together in many ways to create a phenotype. We use flower pigmentation as a phenotype examples to
discuss these alternative pathways.
A. Two Genes have the Same Basic Effect.
A1. Either gene alone yields the full phenotypic effect.
A dominant allele of either gene yields full yellow pigment.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aABB
aAbb
aabB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
15 genotypes yield yellow flowers; 1 genotype yields white
A2. Either gene alone yields a partial phenotypic effect; together
they yield an enhanced effect.
A dominant allele of either gene yields lavender pigment, and a
dominant allele of both genes yields darker, purple pigment.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aABB
aAbb
aabB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
9 genotypes yield purple flowers; 6 yield lavender; 1 yields white
B. Two Genes work in succession on a single path.
B1. A dominant allele of 1 gene creates an intermediate product.
A dominant allele of the other creates the new phenotype.
A dominant allele of A creates an intermediate product and a
dominant allele of B acts on this product to create blue pigment.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aABB
aAbb
aabB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
9 genotypes yield blue flowers; 7 genotypes yields white
B2. A dominant allele of 1 gene creates a phenotype. A dominant
allele of the other acts on this product to create a phenotype.
A dominant allele of A creates a product with orange pigment; a
dominant allele of B acts on this product to create red pigment.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aABB
aAbb
aabB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
9 genotypes yield red flowers; 4 yield white; 3 yield orange
C. One Gene Blocks Expression of Another Gene.
C1. One gene fully blocks expression of a second gene.
A dominant allele of A yields pink pigment, but a dominant
allele of B blocks all expression of A.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aAbb
aABB
aabB
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
13 genotypes yield white flowers; 3 genotypes yields pink
C2. One gene acts on a precursor phenotype to create a new one.
The other gene blocks formation of the precursor phenotype.
A dominant B allele acts on a yellow pigment to form orange.
A dominant A allele blocks formation of the yellow precursor.
AABB
AAbb
aaBB
aabb
AaBB
Aabb
aaBb
aabB
aABB
aAbb
AABb
AaBb
aABb
AAbB
AabB
aAbB
9
3
3
1
A-BaaBA-bb
aabb
12 genotypes yield white flowers; 3 yield yellow; 1 yellow