10 - Uses of human polymorphisms Flashcards
Why creating a human genetic map necessary? (2 reasons)
Scientists wanted to build up maps of how genes were arranged on our chromosomes.
Vital in order to:
- Enable the mapping and identification of disease genes
- Tackle the human genome sequence project
What are the three factors that are termed linkage in genes?
Linkage:
- Whether one locus allele influences another locus allele
- A measure of how far apart genes are from each other.
- How likely a meiotic recombination event is between two loci.
How is genetic mapping of polymorphisms without corresponding phenotypes possible?
Newer molecular biology technologies are able to use polymorphisms without corresponding phenotypes to be used as genetic markers.
Technologies such as RFLPs, DNA sequencing, gel electrophoresis and PCR.
How is linkage of alleles of two microsatellite loci determined?
If the alleles of two microsatellite loci are found in relatives to be:
Always inherited together - complete linkage
Mostly inherited together - partial linkage
Inherited together 50% of the time - no linkage.
What is meant by phase and haplotypes and how can they be determined from raw genotyping data?
Phase - on the same chromosome
Haplotype - Order of in-phase alleles along chromosome.
Using raw genotyping data (mum, dad and childs alleles) the phase can be assigned as the haplotypes are built up
How are linkage maps created?
Linkage maps are based on determining the relative position of genetic markers by the frequency of marker separation during meiosis through generations.
Meiotic recombination drives the study.
How is linkage measured?
Likage is measured in cM
1cM = recombination fraction of 1/100 meiosises.
1 cM is a 1% chance of recombination occurring between two markers.
1 cM is physically equivalent to between 0.7 and 1 Mb of DNA - this depends on the recombination frequency at that part of the chromosome.
How is linkage used to locate disease genes and what are three disease genes that have been located in this way?
Sometimes there is a link between SNPs and disease genes
by finding a linkage between an SNP and a disease gene the location of the disease gene is easier to pinpoint.
Disease genes located using linkage:
- Cystic fibrosis
- Huntington’s disease
- Breast cancer.
What is an issue with using linkage to identify disease genes?
It only provides a rough location of the disease gene.
Why is DNA cloned into genomic DNA libraries?
A whole genome is too complex to work with so DNA strands are chopped up and stored to make a genomic DNA library.
The chromosomes can then be re-assembled by arranging the individual clones into their correct order.
How are segments of the human genome cloned and why is the cloning necessary?
Bacteria’s plasmids can be engineered to carry bits of foreign DNA (segment of human genome).
The bacteria become photocopiers - producing infinite quantities of identical copies (clones) of that DNA.
The large quantities of DNA are needed for analysis such as sequencing.
What are the 4 steps in creating a DNA clone library?
- Break up the human genomic DNA into small pieces using random physical shearing or restriction digest.
- Place each fragment into a vector which is a DNA sequence that allows its selection and replication when introduced into a prokaryotic species.
- Store these in bacteria (BACs) or yeast (YACs) as clone library.
- A single bacterium with one (BAC) in it will grow into a clonal colony that can be further propagated, stored in a freezer or have the BAC + genomic DNA fragment isolated for analysis.
How are the DNA clone libraries converted into a physical map?
BAC/ YAC clones used as probes for FISH - tells you where they are located on which chromosomes.
A contig is made - a virtual stretch of genomic DNA made up from the analysis of multiple overlapping DNA clones.
Eventually the complete genome can be reconstructed in the clones.
Why is having a physical map beneficial? (2 reasons)
The physical map provides access to the precise chromosome DNA stretch that:
- Contains a gene associated with a particular genetic disorder.
- Is a convenient unit of the human genome that can be sequenced.
