Lecture 8 Flashcards
what is the purpose of a forward genetic screen?
to identify genes involved in a biochemical pathway
what is an auxotroph?
unable to synthesize specific compound required for growth
describe 4 steps of forward genetic screen
- use X rays to mutagenize WT conidia
- cross with WT conidia
- isolate spores and culture in minimal media
- add in different compounds to media to see what allows for growth / i.e. to identify mutant
what does this table show? if the pathway is:
1. precursor
2. ornithine
3. citrulline
4. arginine
arg-1: supplementing with all compounds allows growth, therefore mutation must be upstream of ornithine
arg-2: doesn’t grow when supplementing with ornithine so mutation must be downstream of ornithine but upstream of citrulline
arg-3: only grows when supplementing with arginine so mutation must be upstream of arginine but downstream of citrulline
how can we study multiple mutant alleles of the same genes?
complementation test
is the complementation test for recessive or dominant mutations?
recessive
describe the complementation test steps and 2 possible outcomes
- randomly mutate
- cross to make heterozygous for 2 mutants
- 2 outcomes:
- each gene appears WT
- gene appears mutant
what does it mean if the complementation test shows WT genes?
the 2 mutations are on 1 allele of 2 diff genes –> since recessive, both genes appear normal
what does it mean if the complementation test shows mutant phenotype?
the 2 mutations are on both alleles of 1 gene –> no WT allele to compensate
if a white mutant flower is crossed with blue WT flower and makes blue progeny, what does this indicate about the dominance/recessiveness of the blue and white trait?
mutant white is RECESSIVE to WT blue
if progeny of white mutant flower crossed with blue WT flower are crossed, what is the ratio we expect if the white mutation occurs in 1 gene only?
expect 3:1 ratio (blue to white)
if diff ratio, whiteness is caused by mutation in >1 gene
if 2 diff white mutant flowers are crossed and the progeny is all white, what does this indicate?
both mutations affect the same gene bc the progeny appear as mutant –> no complementation
if 2 diff white mutant flowers are crossed and the progeny is all blue, what does this indicate?
the 2 mutations affect diff genes bc the progeny appear WT –> WT allele compensates for the mutation
what is the genotype ratio for 2 genes on same pathway and for regulator gene w target gene?
explain
9:7
normally, we expect 9:3:3:1
- for 2 genes on same pathway: if further upstream gene is WT but downstream gene is mutant (3), phenotype will look the same as the other 3+1 –> makes 7
- for regulator and target genes: if regulator gene is mutated, it cannot function to activate the target gene so appears the same as if target gene is mutated and if both regulator and target gene are mutated
what is recessive epistasis?
recessive genotype of 1 gene masks the phenotype associated with another gene
phenotype ratio found in recessive epistasis?
9:3:4
how is this an example of recessive epistasis?
is w epistatic to m or is m epistatic to w?
even tho m is normal, normal w is required to be able to function so if there’s w mutant it will appear the same both w and m are mutated
w is epistatic to m
gene 1: makes blue pigment in specific cell
gene 2: required for survival of pigment-producing cells
what is this an example of? explain
recessive epistasis
mutation in gene 2 means gene 1 cannot function –> gene 2 is epistatic to gene 1
what is a gene suppressor?
mutant allele of a gene that REVERSES the effect of an original mutation on another gene to allow normal activity of the original gene
example of gene suppressor in context of 2 proteins that must form complex to be active
gene 1 is mutated –> cannot interact with gene 2 and cannot form active complex
then gene 2 can mutate so it can form active complex with mutated gene 1
what happens if a suppressor mutation occurs without an original mutation?
improper function
what is a gene modifier?
mutant allele of a gene that changes the DEGREE OF EXPRESSION/phenotype of a mutated gene
when are gene modifiers common?
common with mutations in regulatory sequences
what is synthetic lethal
mutations in 2 genes, each with weak phenotype that can lead to lethality (i.e. single mutant in protein complex is fine but double mutant = non-functional
