Signatures Of Selection Flashcards
Truncation
Only individuals in a population larger than a certain threshold can reproduce
Negative selection
The most desirable trait remains and is dominant over the less desirable (bell curve graph)
Positive selection
One trait increases in frequency as is desired
Skewed graph
Balancing selection
There is equal frequency across all traits in a population
What does a positive tajimas D result suggest?
Balancing selection or a bottleneck (more pairwise differences than segregating sites)
What does a negative tajimas D result suggest?
Positive selection within the population (more segregating sites than pairwise differences)
Types of quantitive traits
- Continuous e.g. height
- Meristic: phenotypes in discrete classes e.g. number of petals on flower
- Discrete: trait either present or not
What’s a liability value?
Traits that are threshold dependent
- need accumulation of genetic and environmental traits
- threshold same for individs but if relative have it then your chances of passing threshold higher
E.g. diabetes
Phenotypic variation equation
Var (P) = Var (G) + Var (E)
Broad sense heritability
H2 = Var(g)/Var(p)
- influence of genes on phenotypic trait
Narrow sense heritability
h2= Var(a)/Var(p)
- how additive genes have influenced the phenotype
The selection differential equation (truncation)
S = us - u
Us: the mean of the desire phenotype pop
U = the mean of the whole population
The selection response equation (truncation)
R = u’ - u U’ = the mean phenotype of the offspring u = the mean of the whole population.
The realised heritability (truncation)
- h2 = R/S
How to tell if the environment is the only driver in phenotype change between two environments (on a chart)
The change is the same across all genes
All lines change in the same way
How to tell if there is a genotype x environment interaction changing phenotypes between states(on a chart)
The lines (genes) cross each other
How to tell if there is conditional neutrality between states on phenotypes
Some of the genes appear to change between states but one or more dont change at all.
How to map loci with recombinant inbred lines
Cross breed 2 parents with differing phenotypes = F1 intermediate
Let F1 self = F2
Let F2 inbreed
After many generations you can see all traits and choose the ones you want to select for
Make molecular markers to see what genes are the same for all chosen individs
Describe interval mapping
Lander and Botstein (1989)
Uses estimated genetic map for the location of QTLs
Intervals are defined by ordered pairs of markers e.g. SNPs
Stats are used to test if a QTL is likely to be in an internal
Results displayed as LOD (logarithm of odds) scores
- high LOD suggests that the loci is present in that interval
No interactions between QTLs considered
What is association mapping?
- alleles that cause disease phenotypes should be v low frequency in a pop
- however after a mutation recombination maintains linkage disequilibrium… the older the mutation the tighter the region
- cross parents with different 2 phenotypes
- see % of progeny that have mixed phenotypes (one from each parent)
That % = map units and centimorgans for distance on chromosome from each other
Describe landscape genomics
Sample individs on grid over a landscape and their genotypes for markers
Record the environment variables e.g. temp
Observe any correlations between environment and their markers to see if its a response to the environment
Us
The mean of the desired phenotype in OG population (mean of cut off range)
u
The mean of the whole OG population
U’
The mean of the offspring population
