Gene discovery in eukaryotes and linkage mapping Flashcards
Please describe what suppresses recombination and why it is important
Supression of recombination keeps combinations of linked alleles together esp. important combinations of alleles that determine particular sexes or mating types hence need to be co-inherited.
recombination is often suppressed by chromosomal inversions wherein a segment of a chromosome is reversed from end to end. can either be paracentric or pericentric.
Recombination in the inverted region results in aberrant chromosomes and failed meioses hence the recombination frequency is much lower.
Why do sex chromosomes have extremely low recombination rates?
Because they often have many inversions
e.g. human X and Y chromosomes
similar in Drosophila
explains why Morgan X-linked traits showed complete sex linkage - unlike linked autosomal traits that had a higher frequency of recombination
Describe forward and reverse genetic approaches
Forward genetic: identify the sequence variation responsible for a particular phenotype. i.e. Phenotype -> sequence variation.
Reverse: Identify the phenotype caused by a particular sequence variation. Sequence variation -> phenotype. tests assumption of gene function
Forward genetic approach. What are the steps
Identify the indivudauls with heritable phenotype of interest
identify the causative DNA variation with the selected method.
What are the differences between natural and induced genetic variations
Natural variations - identified in a population (disease resistance, herbicide resistance, fur )
Induced - chemical mutagens, point mutations, x rays / gamma rays - deletions, transposable elements - inversions
What are the 4 methods to identify mutant genes in eukaryotes?
Molecular complementation - yeast
insertion mutagenesis - drosophila and plants. use of transposons / transposable elements. found in eukaryotes and prokaroytic genomes. mobile , selfish DNA sequences . replicate themselves and insert new copies randomly in genome causing mutations.
Linkage mapping!
Whole genome sequencing - only in organisms with complete genome sequence
Linkage mapping of mutant gene X. How would you go about doing this?
Linkage is measured based on co-segregation of mutant phenotypes.
maximum measured frequency of recombination is 50%.
What are genetic markers
Loci with different allies at the known genetic position that serve as reference points in the genome
What do we need for mapping a mutation?
Parents with different genotypes - allele combinations at the gene of interest AND genetic markers
- we can detect different alleles
progeny with segregating allele combinations (genotype)
at the gene of interest AND genetic marker position(s)
detect linkage and recombination between gene of interest and genetic markers
Linkage mapping
Phenotype based determination of genotype limits number of genetic markers
What is a molecular marker
Is a site of DNA polymorphism that can be detected with molecular techniques.
not associated with any observable phenotype
each sequence variation is an allele
naturally occurring variations
very numerous
can be used to mapping - if different allies are present in homozygote form in padres
What are the types of variation for molecular markers?
Short insertion or deletions - indels
single nucleotide polymorphisms - SNPs
Indels
Variation in the number of short sequence repeats
detected by PCR
SSLPs simple sequence length polymorphisms
detected by agarose gel electrophoresis
molecular markers are genetically co-dominant
both alleles are equally strong and can be visible in a heterozygote . any genotype can be distinguished in a individual
SNPs
Substitution of a single nucleotide sometimes affects a restriction endonuclease site CAPS markers (cleaved amplified polymorphic site) detected by PCR and restriction endonuclease digestion
Linkage mapping with molecular markers
A progeny with segregation between the mutant gene and multiple molecular markers , usually molecular markets are naturally occurring DNA polymorphism between individuals
we ned to know DNA variations in the individual carrying the mutation - well characterised WT strains are used for mutagenesis.
cross generates individuals for the mapping population
part of the progeny used for mapping the gene
