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Practice Problem 2: dominance and epistasis
You have access to a novel reporter that is sensitive to reactive oxygen species (ROS) levels, and it
flashes in response to levels of ROS (intensity of flashes corresponds to ROS levels) in living animals. The
undergraduate in your lab has already performed a screen for genes important for generation of ROS,
and the homozygous mutant phenotypes are shown below (ft1-ft4). (The phenotypic name of your
mutants is ros; your allele designation is ft.)
Strain:
WT
ft1
ft2
ft3
ft4
Flash intensity 100
30
40
50
60
You cross WT males with each of your homozygous mutant ft hermaphrodites and obtain the following
phenotypes in F1:
Strain:
WT
ft1
ft2
ft3
ft4
Flash intensity 100
100
40
100
75
What can you conclude regarding the dominance of each allele?




ft1:
ft2:
ft3:
ft4:
recessive
fully dominant
recessive
semi-dominant (phenotype is intermediate between wt and mutant)
What further crosses can you do to further test the dominance of your alleles?


Cross both ft2 and ft4 homozygotes to a null or a deficiency (ft2/df or f24/df)
i. hypermorph- lessens the phenotype
1. Hypermorph- produces increased activity or levels of yfg, also refered to
as “gain of function”
ii. neomorph- the same
1. Neomorph- a mutant of yfg with a novel, gain of function phenotype
iii. antimorph- worsens the phenotype
1. Antimorph or Dominant negative- the product neutralizes the wild-type
product of yfg
iv. haplo-insufficient- worse or possibly dead
1. Haplo-insufficiency- loss of one copy of yfg is not enough gene product
to show wild-type function.
Can also look at df/+ : if it’s wt, then you know that the phenotype is not due to
haploinsufficiency
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The hermaphrodite mutants could have selfed instead of mating with the WT males. How would
this affect your dominance assignments?

If hermaphrodite mutants selfed instead of mating with WT males, recessive mutations
would look dominant because the F1 progeny would also be recessive and would display
the mutant phenotype. This would mimic the results of a dominant allele since it would
show up in the F1 progeny after crossing to WT males.
You make the remarkable discovery that in response to a 5 second flash of blue light the animals curl up
in to a little ball. This is a robust, reproducible response. To identify genes important for this response,
you perform an F1/F2 mutagenesis screen and identify crl mutants. They fall into 2 categories:


those that curl even in the absence of blue light: crl-1, crl-2
those that fail to curl in response to blue light crl-4, crl-7, crl-5, crl-6
All crl mutants are recessive, single gene mutants.
You obtain the following results when analyzing double/triple mutants:
(-) flash of blue light
(+) flash of blue light
WT
does not curl
curls
crl-1-/-; crl-5-/-:
curls
curls
crl-1-/-; crl-6-/-:
does not curl
does not curl
crl-1-/-; crl-7-/-:
curls
curls
kcrl-2-/-; crl-4-/-:
curls
curls
crl-2-/-; crl-5-/-:
curls
curls
crl-2-/-; crl-7-/-:
curls
curls
crl-2-/-; crl-6-/-:
does not curl
does not curl
crl-1-/-; crl-2-/-:
curls
curls
crl-1-/-; crl-2-/-; crl-6-/-:
does not curl
does not curl
Draw a genetic pathway that could explain these results. Note ambiguities.
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crl-7
crl-5
crl-1
crl-2
crl-6
curl
crl-4
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