Lecture 8

Question 1

what is the purpose of a forward genetic screen?

Answer 1

to identify genes involved in a biochemical pathway

Question 2

what is an auxotroph?

Answer 2

unable to synthesize specific compound required for growth

Question 3

describe 4 steps of forward genetic screen

Answer 3

1. use X rays to mutagenize WT conidia
2. cross with WT conidia
3. isolate spores and culture in minimal media
4. add in different compounds to media to see what allows for growth / i.e. to identify mutant

Question 4

what does this table show? if the pathway is:
1. precursor
2. ornithine
3. citrulline
4. arginine

Answer 4

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

Question 5

how can we study multiple mutant alleles of the same genes?

Answer 5

complementation test

Question 6

is the complementation test for recessive or dominant mutations?

Answer 6

recessive

Question 7

describe the complementation test steps and 2 possible outcomes

Answer 7

1. randomly mutate
2. cross to make heterozygous for 2 mutants
3. 2 outcomes:
   - each gene appears WT
   - gene appears mutant

Question 8

what does it mean if the complementation test shows WT genes?

Answer 8

the 2 mutations are on 1 allele of 2 diff genes –> since recessive, both genes appear normal

Question 9

what does it mean if the complementation test shows mutant phenotype?

Answer 9

the 2 mutations are on both alleles of 1 gene –> no WT allele to compensate

Question 10

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?

Answer 10

mutant white is RECESSIVE to WT blue

Question 11

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?

Answer 11

expect 3:1 ratio (blue to white)

if diff ratio, whiteness is caused by mutation in >1 gene

Question 12

if 2 diff white mutant flowers are crossed and the progeny is all white, what does this indicate?

Answer 12

both mutations affect the same gene bc the progeny appear as mutant –> no complementation

Question 13

if 2 diff white mutant flowers are crossed and the progeny is all blue, what does this indicate?

Answer 13

the 2 mutations affect diff genes bc the progeny appear WT –> WT allele compensates for the mutation

Question 14

what is the genotype ratio for 2 genes on same pathway and for regulator gene w target gene?

explain

Answer 14

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

Question 15

what is recessive epistasis?

Answer 15

recessive genotype of 1 gene masks the phenotype associated with another gene

Question 16

phenotype ratio found in recessive epistasis?

Answer 16

9:3:4

Question 17

how is this an example of recessive epistasis?

is w epistatic to m or is m epistatic to w?

Answer 17

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

Question 18

gene 1: makes blue pigment in specific cell
gene 2: required for survival of pigment-producing cells

what is this an example of? explain

Answer 18

recessive epistasis

mutation in gene 2 means gene 1 cannot function –> gene 2 is epistatic to gene 1

Question 19

what is a gene suppressor?

Answer 19

mutant allele of a gene that REVERSES the effect of an original mutation on another gene to allow normal activity of the original gene

Question 20

example of gene suppressor in context of 2 proteins that must form complex to be active

Answer 20

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

Question 21

what happens if a suppressor mutation occurs without an original mutation?

Answer 21

improper function

Question 22

what is a gene modifier?

Answer 22

mutant allele of a gene that changes the DEGREE OF EXPRESSION/phenotype of a mutated gene

Question 23

when are gene modifiers common?

Answer 23

common with mutations in regulatory sequences

Question 24

what is synthetic lethal

Answer 24

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