Download Lecture#20 - Gene Interactions and Epistasis

6/24/17
BIOLOGY 207 - Dr. Locke
Lecture#20 - Gene Interactions and Epistasis
Required readings and problems:
Reading: Open Genetics, Chapter 6.2
Problems: Chapter 6
Optional
Griffiths (2008) 9th Ed. Readings: pp 239-248
Problems: 9th Ed. Ch. 16: 26-48, 54-66
Campbell (2008) 8th Ed. Readings: Concept 14.3
Concepts:
How do alleles at different gene loci interact in diploids?
1. Phenotypic expression of some alleles varies due to "genetic background"
or other causes (penetrance, epistasis, genetic suppression).
2. The interaction of genes can modify the typical Mendelian ratios of phenotypic
classes of the progeny.
3. Penetrance and expressivity measure frequency and intensity (respectively)
of phenotypic expression of a particular genotype.
Biol207 Dr. Locke section
Lecture#20
Fall'11
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Genetic Background
The phenotypic expression (mutant phenotype) of a mutant allele can be modified by
different alleles at other gene loci.
Why do some characters vary in their expression?
1. Biological systems are complex
-> slight differences in the expression of one or more genes in the system
-> variation in mutant phenotype
2. Background effects can be due to one, a few, or due to many genes.
3. Phenotypic expression that depend upon the combined effect of a large number of
genes is called the genetic background
-> additive effect of all the allelic differences in the remainder of the genome.
Example of one gene:
The case of
Biol207 Dr. Locke section
recessive epistasis.
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Phenomenon of Recessive Epistasis
Recessive epistasis occurs when
the phenotypic expression of genotypes at one locus
depends on the alleles at another locus.
Recessive epistasis can alter phenotypic ratios.
E.g. - not 9:3:3:1 (F2 cross) or not 1:1:1:1 (test cross).
Example of recessive epistasis: “C” gene in mouse coat colour
There are many loci that affect coat colour in mice.
B/b locus B = Black pigment b = brown pigment
C dominant = permits coat colour expression (of other gene affecting coat colour)
c recessive = prevents coat colour expression
cc, homozygous recessive lack any coat pigmentation - called albinos - white hair
Because they have hair with no pigment, cc prevents (precludes) the expression of
any other hair colour loci
Labrador retrievers coat colour
B/b locus B = Black pigment b = brown pigment
E/e locus E = permit deposition e = prevent deposition
Biol207 Dr. Locke section
Lecture#20
Fall'11
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Example Crosses:
Mice: Interaction between the
B locus and C locus in
(albino) (brown)
BBcc x bbCC P1
BbCc (Black) F1
Cross F1 x F1
F2:ratios
modified ratio
B_C_ black
9
bbC_ brown
3
B_cc albino-white
bbcc albino-white
4
Result:
The cc homozygote is said to be
epistatic to the other coat colour
genes
The albino phenotype prevents one
from "seeing" the phenotype from
the other loci
Dogs: Interaction between the
B locus and E locus in
(yellow) (brown)
BBee x bbEE P1
BbEe (Black) F1
Cross F1 x F1
F2:ratios
modified ratio
B_E_ black
9
bbE_ chocolate
3
B_ee yellow
bbee yellow
4
Result:
The ee homozygote is said to be
epistatic to the other coat colour
genes
The yellow phenotype prevents one from
"seeing" the phenotype from the other
loci
Opposite of epistatic is hypostatic
Bb locus is hypostatic to the cc alleles.
Biol207 Dr. Locke section
Lecture#20
Fall'11
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Modified Phenotypic Ratios produced by 2 gene interactions.
Ratio:
Type genotype:
None
9
3
3
1
A-B- A-bb aaB- aabb Ratio
9
3
3
1
9:3:3:1
AB
Ab
aB
ab
Recessive epistasis
9
3
4
9:3:4
of aa acting on B and b alleles
AB
b
a
Dominant epistasis
12
3
1
12:3:1
of A acting on B and b alleles
A
aB
ab
Duplicate genes
15
1
15:1
A
a
Complementary genes
9
7
9:7
A
a
Recessive suppression
9
3
4
13:3
by aa acting on bb
B
b
B
Dominant suppression
15
1
15:1
by A acting on bb
B
b
Shading represents combined classes.
See extra page on modified Mendelian ratios:
http://www.biology.ualberta.ca/courses.hp/bio207.hp/locke/modified_ratios.htm
Biol207 Dr. Locke section
Lecture#20
Fall'11
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Genetic suppression
Suppressor mutation has the effect of suppressing (reducing) the phenotypic
expression of a mutation -> results in a more wild type (less mutant) phenotype.
Can be:
- intra-genic (same allele) - similar to a revertant
- extra-genic (different gene locus)
Dominant Suppression
Example: white-mottled allele (wm) gives
mosaic expression of white
wm / wm = white mottled phenotype
wm / w+ =
wm / w- =
Dominant Suppressors of wm :
wm / wm ; Su- / +
or
wm / wm ; Su- / Su-
Biol207 Dr. Locke section
= w+ phenotype
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Cross:
w+/w+; Su-/Su-
X
wm/wm ; Su+/ Su+
P1
w+/wm; Su/Su+
w+ / _
wm/ wm
w+ / _
wm/ wm
; Su- / _
; Su- / _
; Su+ / Su+
; Su+ / Su+
F1
9
3
3
1
red
red
red
mottled
15
F2
1
Note: similar ratios to gene duplication (also 15:1 ratio)
Enhancer mutations
Opposite of suppressors
-> they make the phenotype more mutant (enhance the mutant phenotype).
Many Genes -> genetic background
A gene acts in conjunction with other genes and with the environment.
Biol207 Dr. Locke section
Lecture#20
Fall'11
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Penetrance & Expressivity
Penetrance - Is it there or not?
Penetrance - The percentage of individuals with a given genotype in a population who
exhibit the phenotype associated with that genotype
Note: It may be that not all individuals with genetype a/a exhibit the a phenotype
because of suppressors, epistatic genes etc., environment
Example: In my lab -> a strain that shows incomplete penetrance of an eye "bleb"
phenotype
Expressivity - It's there, but how strong is it?
Expressivity - the variation in the extent to which a given genotype is expressed
phenotypically in a individual or population
From my lab: Drosophila wing mutant has a variably expressive phenotype
- interruption of a vein in the Drosophila wing
- variable loss of the vein
There is fly-to-fly variation in the extent to which this
character is expressed
A range of expression in the population.
complete
20%
Biol207 Dr. Locke section
partial
40%
little
30%
no loss
10%
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Wild type
incomplete penetrance
Variable expressivity
mutant
complete penetrance
constant expressivity
Variable penetrance and expressivity.
Biol207 Dr. Locke section
Lecture#20
Fall'11
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