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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 1 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. 2 crl-7 crl-5 crl-1 crl-2 crl-6 curl crl-4 3