The Genetics of Complex Human Traits Flashcards
What is the definition of heritability?
The degree of variation in a phenotypic trait that can be explained by genetic variation
Scale of 0-1
Measure of VARIATION
How can the heritability of a trait be determined?
Heritability Studies
Parent-Offspring Correlations
How is a heritability study carried out?
Usually uses dizygotic or monozygotic twins
Look at the concordance rate of a trait (presence of same trait in both members)
If the concordance rate for a trait is higher in monozygotic twins than in dizygotic twins, it means there is a greater genetic component
What are parent-offsping correlations?
Plot the average of the two parents phenotypes against the offspring phenotype
Forms a scatter graph
The stronger the correlation, the higher the heritability
What is the most common type of genetic variation?
SNPs
How often do SNPs occur?
Every 100-300 bases
There are 10 million SNPs in the human genome
How do SNPs affect inherited traits?
Certain SNPs can cause a predisposition to a disease - do not cause disease
People with more SNPs for a high BMI will be heavier on average
SNPs have an additive effect - some have a greater weighted effect than others
What are candidate gene studies?
Old method of identifying genes/SNPs involved in disease
Identify a certain gene
See if that particular gene correlates with occurrence of a certain disease
What are genome wide association studies?
Current method of identifying SNPs associated with complex traits
Uses SNP chips to read genomes
Compare prevalence of SNPs in a case group to a control group
Identifies areas of the genome that may be associated with disease
What are 2 benefits of GWAS?
Identify important genes in disease, that previously weren’t thought to be important
Develop personalised medicine based on an individual’s genotype
What are 3 limitations of GWAS?
- Some SNPs have such a small additive effect that they are not helpful for predicting a disease
- If multiple SNPs lie within the same gene, it is hard to tell which one is responsible for causing the disease
- Missing heritability
What is missing heritability?
For a trait that has very high heritability, such as height, why have we not identified SNPs that contribute towards all the inheritance? The explained heritability is less than we would expect
What are some possible causes for missing heritability?
- Overestimation of heritability: These traits are not actually as heritable as first thought
- Genetic Interactions: Genes do not have an additive effect towards traits, but rather interact more complexly with eachother
- Sample size. Larger sample sizes are needed to detect SNPs with very small effects.
- Rare variants and Structural variants that are harder to identify but have a huge effect
Why is a GWAS ‘hypothesis free’ research?
Researchers don’t start out with a hypothesis that a certain gene is involved
Carry out analysis of ALL genes and look for correlations
What is relative risk?
Risk compared to the population
What is absolute risk?
Individual risk
What is a haplotype?
A pair of alleles that are closely linked on a chromosome, so are often inherited together.
Conserved sequence that survives many generations
What is linkage disequilibrium?
When the frequency of association between two genes is higher or lower than what would be expected if they were independent from eachother, and randomly assorted
What is linkage analysis?
Detection of the chromosomal location of genes, using the fact that genes that are located closer together are less likely to become separated during recombination
How is linkage measured?
LOD score = how likely two genes are to be linked
A LOD score > 3 suggests linkage between the genes
What is a complex trait?
A polygenic trait; the phenotype is a result of variation in many genes, and interaction with environmental factors
Name 3 differences between monogenic traits and complex traits?
Monogenic = single gene, complex traits = many genes
Monogenic traits are rare where as complex traits are very common
Monogenic traits follow mendelian inheritance, complex traits are very difficult to predict
2 examples of monogenic traits
Dwarfism
Earwax Type
What is SNP genotyping?
Analyse DNA sequence of two people using a biological assay
Measuring the variation in SNPs between members of a species
How is proportion calculated for a single group?
(Number of participants in category)/(Total number of participants)
How is the odds of having the disease calculated for a single group?
(Number of participants in disease category)/(Number of participants in other category)
OR
(Proportion of participants in disease category)/(Proportion of participants in other category)
How is the odds ratio calculated between two independent groups?
(Odds of having disease in one group)/(Odds of having disease in the other group)
What is a susceptibility variant?
A variant that is associated with a disease - not causal
Why don’t GWAS find causal genes for diseases?
Due to the presence of haplotypes (pairs of SNPs often inherited together), GWAS often identify a variant that is closely associated with the causal gene. Therefore they implicate areas associated with disease, not specific genes
Example of a disease with complex inheritance
Alzheimer’s disease
What is Alzheimers?
Neurodegenerative disorder of the CNS
Caused by formation of plaques on the brain
Symptoms are cognitive decline and memory loss
What is the heritability of Alzheimers?
49-79%
What is familial Alzheimers?
Inherited autosomal dominant condition
Onset before age 65
About 0.1% of cases
