Genetics 1- Genetic drift

Genetics 1- Genetic drift

1. Definitions

   • Hardy Weinberg theory- Null expectation for expectation of genotype frequencies in a population
   • Wright Fisher- Models for how genetic drift and selection affect populations
   • Allele- different states of a locus
   • Haplotype- Allelic state which occurs across multiple loci
   • Selection- Difference in fitness between different genotypes

2. Hardy Weinberg equilibrium theory

   • Calculate genotype freqs that we would expect to see in a pop for a given set of allele freqs
   • If genotype proportions of a succeeding generation are found to be indistinguishable from those in parental generation--- no evolution has occurred
   • Assumptions
     • Diploid organisms
     • Infinite pop size- No strong genetic drift that might affect allele freqs
     • Independent segregation of alleles
     • Random mating
     • Equal number of offspring per parental allele
     • No migration
     • No mutation or selection
   • Deviation from equilibrium-- one assumption is not being met
     • Non-random mating-- particular genotypes more likely to mate
     • Pop substructure-- allele freqs in subpops won't be calculated correctly

3. Genetic drift- randomness or chance variation

   • Variation in allele frequency between generations can occur solely through process of sampling-- each generation is only a finite sample from the previous one
   • Reduces variation-- causes alleles to become fixed or lost
   • Selection
     • Positive selection
       • Increases freq of an allele
       • Reduces amount of genetic variation
     • Negative selection
       • Removes deleterious alleles from pop
       • Reduces genetic variation
     • More likely to occur in larger pops
   • Unlike selection-- each allele in parental pop is affected in the same way
   • Drift selects all DNA-- whether selection or neutral
   • More likely to occur in smaller pops
   • Wright-fisher model-- null model of no selection-- only drift is affecting genotype freqs
     • Assumptions
       • Non-overlapping of generations
       • Constant size pop
       • Random mating-- 2 parents
       • Sexual reproduction-- all individuals are hermaphrodites- able to self-fertilise
     • Effective pop size= Ne
       • Unlikely that a natural pop will conform exactly to assumptions of wright-fisher
       • Real pops behave in similar way to wright-fisher pops- but with reduced pop size
       • Ne--- genetically effected pop size
       • Ne< real pop size
       • 10% of real pop size
       • Smaller effective pop size-- higher the chance of drift
     • Small population-- lots of fluctuations in allele size
   • If drift was the only evolutionary force active-- pops would be homozygosity--- all alleles would descend from a single allele
   • Maths
     • Probability of fixation= 1/2N= freq in population
     • F= probability of identity of any 2 alleles--- Sum of (P^2)-- sum of allele freqs squared
       • As time goes on genetic drift creates increasing amounts of shared ancestery-- F value goes up
       • F value goes towards 1---- alleles go towards fixation
   • Coalescence theory
     • TMRCA= Time to the most recent common ancestor
     • Probability that 2 samples are descended from the same parental allele in one generation= 1/2N
     • Probability that 2 samples are independent in the line of descent= 1- 1/2N
     • Probability that 2 samples will coaslesce in generations t (t= generation no.)= (1- 1/2N)^r-1 x 1.2 N