Gene Discovery and Gene Mapping in Eukaryotes Flashcards
What are DNA polymorphisms
DNA sequence variations among individuals.
Name two types of DNA polymorphisms.
SNPs (Single Nucleotide Polymorphisms) and Indels (Insertions and Deletions).
Why are DNA polymorphisms useful in genetics
They can be linked to phenotypes, helping identify genes important for biological processes.
What is forward genetics
Approach: phenotype → sequence variation; no prior gene function assumption.
What is reverse genetics
Approach: sequence variation → phenotype; tests a hypothesis about gene function.
What are the two main steps in forward genetics
Isolate mutant phenotype and identify causative DNA variation.
What are natural genetic variations
Polymorphisms already present in populations (e.g., disease resistance, fur color).
What are induced mutations
Mutations caused by external agents like chemicals or radiation.
What does EMS (ethyl methanesulfonate) cause
Point mutations, especially C→T or A→G.
What mutations do UV light typically induce
Point mutations.
What type of mutations do X-rays or gamma rays cause
Deletions.
What role do transposable elements play
They insert randomly into the genome and can disrupt gene function.
What are transposons
“Selfish” mobile DNA elements that can replicate and insert themselves randomly.
Who discovered transposable elements and in what organism
Barbara McClintock, in maize.
Why is insertion mutagenesis useful
If TE sequence is known, it can help identify and clone the disrupted gene.
What limits insertion mutagenesis
Low efficiency, mostly causes loss-of-function, and works mainly in model organisms.
Name three methods to identify mutant genes.
Insertion mutagenesis, linkage mapping + map-based cloning, and whole genome sequencing.
Why can’t we just sequence the genome to find mutations
Too many polymorphisms; need positional information to know which one is causative.
How frequently does EMS induce mutations in Drosophila
~1 mutation every 150–300 kb (~1 in every 30 genes).
What is recombination frequency
The proportion of recombinant progeny among total offspring.
What is the relationship between gene distance and recombination
Greater distance = higher recombination frequency.
Who developed the concept of linkage mapping and when
Morgan and Sturtevant, 1911.
What is 1 map unit (MU)
A 1% recombination frequency = 1 centiMorgan (cM).
What’s the maximum recombination frequency
50% — indicates genes are unlinked.
What’s the minimum recombination frequency
0% — complete linkage.
What does a genetic map measure
Relative gene positions based on recombination (in cM).
What does a physical map measure
Actual distances in base pairs (bp).
Why are genetic and physical maps different
Because recombination rates vary along the chromosome.
Where is recombination typically low
Near centromeres.
What are recombination ‘hot-spots’ and ‘cold-spots’
Areas with unusually high or low recombination frequency, respectively.
What is crossover interference
One crossover event can reduce the likelihood of another nearby crossover.
Why do genetic distances underestimate physical distances above ~30cM
Multiple crossover events can occur, but aren’t all detected.
What does ~50% recombination indicate
Genes are on opposite ends of a chromosome or unlinked.
What are molecular markers
Polymorphic DNA sites not linked to a visible phenotype, used for mapping.
What makes a good molecular marker
Detectable variation between parental genotypes, and dense across the genome.
How do you map a mutation like gene M
Track recombination between mutant phenotype and molecular markers.
What indicates a recombination event in mapping populations
Presence of the opposite allele of a linked marker in a homozygous mutant.
