Genetic analysis

Question 1

Why is mutagenesis screening important

Answer 1

• don’t have to know underlying biochemical or physiological systems or what molecules are involved
• can find many genes from one screen that can all be catagorised
• mutants can be used to find genes with related or interacting functions

Question 2

Carrying out a complementation test

Answer 2

i. identify lots of mutants that are different to the wild types
ii. cross all white eyed mutant with wild type, to see if recessive/dominant
iii. cross all mutants with each other

(if mutations not X-linked reciprocal test not needed)

must have recessive mutants

Question 2

Solving the complementation test

Answer 2

• when two mutants cross and the recessive trait is seen those mutants are in a complementation group
• number of mutants in complementation group = number of alleles in the group
• number of complementation groups = number of genes

Question 3

Allelic series

Answer 3

• a collection of mutant alleles for a gene, they are different mutations in the same gene
• each allele can have different gene activity leading to slightly different phenotypes
• CF (cystic fibrosis) is a recessive genetic disease caused by mutations in CFTR gene some mutations result in partial or residual CFTR activity and the phenotype of patients varies widely because of the different alleles

Question 4

Allelic series CF

Answer 4

the different alleles causes different symptoms

• 50% of CF patients are homozygous for the same mutation and other alleles are much rarer
• 95% of patients present common phenotype even though they carry different alleles (abnormal sweat electrolytes, sino-pulmonary disease, male infertility, pancreatic exocrine insufficiency)
• some patients presenting male infertility, sino-pulmonary disease or intestinal obstruction also have CF but are more difficult to diagnose
• the phenotype of CF patients is also determined by genetic modifiers and the environment

Question 5

How can mutants be used to find other genes with similar or related functions

modifiers

Answer 5

• screen for new mutations in mutant background (modifier screen detects two mutations)

i. suppress of original mutant phenotype: suppressors
ii. enhance original mutant phenotypes (more extreme phenotype): enhancers

Question 6

What are the modifiers in C.elegans

Answer 6

dumpy mutants have mutations in collagen genes, these encode components of collagen or enzymes in collagen biosynthetic pathway, mutations in dumpy genes can lead to death in embryonic or larval stages.

Question 7

Modifier mutants

Answer 7

• some modifier mutants are aphenotypic (same phenotype as wild-type)

Mendalian cross ratios can change with this not showing expected 9:3:3:1 ratio

must consider biological context when working out ratio

Question 8

How do you work out a biosynthetic pathway

Answer 8

• work out the number of genes (each arrow is a gene)
• find wild-type (starting)
• work out what must change to get new phenotype

eg.
orange x yellow
F1 progeny: red
F2 progeny: 9 red: 4 yellow: 3 orange

yellow —> orange —> red
y o