Lecture 8

Question 1

What’s the one gene one enzyme hypothesis

Answer 1

Theory that one gene produces an enzyme and that that enzyme is in charge of one step in a pathway, not the whole pathway

Question 2

What are forward genetics

Answer 2

When you hit an organism or sample with UV or X-rays to purposefully get mutations so as to be able to study it

Question 3

If we put a mutated spore on complete medium and get growth and then on a minimal medium and get no growth what does that tell us

Answer 3

Tells us that theres some nutrient found in the complete medium that the cell itself cannot produce, like the neurospora that cant produce arginine, auxotroph
It survives if its in the environment but not if we reduce it to the basic basic building blocks

Question 4

How can we determine what element of the complete medium the organism cannot synthesize

Answer 4

You slowly add things back, trying different combinations until you isolate what in the complete medium makes it capable of survival
Could be vitamins, amino acids, minerals, etc

Question 5

What’s the purpose of the complementation test

Answer 5

Its a way to figure out whether two recessive mutations that affect the same phenotype are in the same gene or on different genes
It’ll distinguish whether its a mutation in the same pathway (thats why resulting in a certain phenotype, but its a fuck up in a different step) or whether its just different alleles of the same gene (mom copy has one mutant, dad has another)

Question 6

Describe the possible results of the complementation test

Answer 6

Say youre crossing a1/a1 with a2/a2 where these are two individuals with the phenotype (in a1 the individual is mutant for gene A but wild for gene B, and for a2 the opposite), you get heterozygous offspring, there are two cases
Case 1: the mutations are on different genes, so a1 is able to contribute a wild type gene/allele B while still being mutant for Gene A, and a2 can compensate for it in the same but opposite way. Result: you get majority wild type offspring
Case 2: the mutations are on the same gene, so we have two different alleles, but both are non-functional so it doesn’t help much
Refer to slide 7

Question 7

Why does a complementation test change the phenotypic ratio of 9:3:3:1 to 9:7

Answer 7

9:3:3:1 now applies to genotype, thats just the distribution
9:7 now describes the phenotype. 9 will be wild type, wild type here meaning that there at least 1 wild type allele for each gene involved (in our case 2), this leading to full compensation/complementation and therefore proper expression. The other 7 will be either completely recessive (1), recessive for one trait (3), or recessive for the other (3)
Being recessive for one trait puts them in the white because failing in one step of the pathway is enough to affect phenotypic expression. Both genes involved in the process need to be compensated for, one isn’t enough

Question 8

What is epistasis, brief recessive epistasis logic

Answer 8

When one gene masks or overrides the effect of another gene
Think of gene A producing an enzyme that converts substrate to X, gene b converting X to Y, gene c converting Y to Z, the final and phenotypically expressed product
If gene A is recessively mutated and cannot make that first enzyme, it unintentionally masks the effect of genes B and C by stopping the pathway prematurely

Question 9

What phenotypic ratio do we get from recessive epistasis and why

Answer 9

9:3:4 ratio, think in context of a sequential process between gene A->gene B-> gene C
9: where all is well such that the dihybrid cross has at least one wild type to cover up the mutant, wild type expressed
3: where the mutation affects gene C, so we never get to the last step, caught in between, a and b still matter
3 and 1: where the mutation affects B so we never get to step 2, stuck at step 1, c doesnt matter but it may be wild type (3) or recessive too (1)

Question 10

What is a suppressor

Answer 10

A mutant allele of a gene that reverses that effect of another mutation (think of it as a mutation that complements a mutation and is able to restore function)
2 mutants that behave like wild type

Question 11

What is a modifier gene

Answer 11

A second mutation that changes the degree of expression of a mutated gene, affects expressivity
Ex: mutation in a regulatory sequence would affect how often the mutated gene is expressed therefore mediating its effects

Question 12

What is a synthetic lethal mutation

Answer 12

Mutations in 2 genes, each with a relatively weak phenotype, that when occurring together result in lethality
Best example is multiprotein complexe