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BIOLOGY 207 - Dr. Locke
Reading: Open Genetics, Chapter 6.2
Problems: Chapter 6
Optional
Griffiths (2008) 9 th Ed. Readings: pp 239-248
Problems: 9 th Ed. Ch. 16: 26-48, 54-66
Campbell (2008) 8 th Ed. Readings: Concept 14.3
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 page 1

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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.
The case of recessive epistasis .
Biol207 Dr. Locke section Lecture#20 Fall'11 page 2

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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 page 3

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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
Opposite of epistatic is
Bb locus is hypostatic to the cc alleles.
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
Biol207 Dr. Locke section Lecture#20 Fall'11 page 4

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Ratio: 9 3 3 1
Type genotype:
None
A-B- A-bb aaB- aabb Ratio
9
AB
3
Ab
3 aB
1 ab
9:3:3:1
Recessive epistasis of aa acting on B and b alleles
Dominant epistasis of A acting on B and b alleles
Duplicate genes
9
AB
12
A
3 b
15
A
3 aB
4 a
1 ab
1 a
9:3:4
12:3:1
15:1
Complementary genes
Recessive suppression by aa acting on bb
Dominant suppression by A acting on bb
9
A
9
B
3 b
15
B
7 a
4
B
1 b
9:7
13:3
15:1
Shading represents combined classes.
See extra page on modified Mendelian ratios: http://www.biology.ualberta.ca/courses.hp/bio207.hp/locke/modified_ratios.htm
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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)
Example: white-mottled allele (w m ) gives mosaic expression of white w m / w m = white mottled phenotype w m / w
+
= w m / w
-
=
Dominant Suppressors of w m : w m / w m ; Su- / + or w m / w m ; Su- / Su- = w+ phenotype
Biol207 Dr. Locke section Lecture#20 Fall'11 page 6

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Cross: w+/w+; Su-/Su- X w m /w m ; Su+/ Su+ P1
w+/w m ; Su/Su+ F1 w+ / _ ; Su- / _ w m / w m ; Su- / _
9
3 red red
15
F2 w+ / _ ; Su+ / Su+ 3 red w m / w m ; Su+ / Su+ 1 mottled 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.
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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 partial little no loss
20% 40% 30% 10%
Biol207 Dr. Locke section Lecture#20 Fall'11 page 8

Wild type mutant incomplete penetrance complete penetrance
Variable expressivity constant expressivity
Variable penetrance and expressivity.
Biol207 Dr. Locke section Lecture#20
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Fall'11 page 9