Population Genetics 6 Flashcards
why don’t we see plants that have equal lengths for both male and female parts?
- to avoid selfing/inbreeding
are sites experiencing selection dependent or independent, why? (2)
- they are dependent
- embedded within genes, within chromosomes, and within genomes
locus
- site or location on a strand of DNA
in the absence of genetic associations, how would we model the frequency of chromosome type?
- it would be the product of the component allele frequencies (frequency of allele A x frequency of allele B = frequency of AB chromosome)
disequilibrium (3)
- symbol
- aka
- definition
- symbol: D
- aka linkage disequilibrium
- measures genetic associations among two loci and indicates which alleles tend to be found together at two loci
what do the values of D tell us (4)
- D = 0 then independent assortment and expected frequencies are given by chance
- D =/= 0 then the alleles carried at the 2 loci are not independent
- D > 0 then positive disequilibrium and there are more AB and ab chromosomes in the population than expected by chance
- D < 0 then negative disequilibrium and there are less AB and ab chromosomes in the population than expected by chance
recombination
- occurs during meiosis in sexual organisms to generate gametes carrying new combinations of alleles
recombination rate (2)
- symbol
- definition
- symbol: r
- determines the probability of a cross-over event that creates non-parental gametes between two loci; a measure of genetic distance
what is the formula for predicting the rate at which parental chromosomes are produced
1 - r; (1-r)/n for each of the parental combinations possible
what is the formula for predicting the rate at which parental chromosomes are not produced
r; r/n for each non-parental combination possible
why do we expect linkage disequilibrium to be found in genomes (4)
- because several processes generate associations along loci: drift, mutation, migration, selection
how does drift generate associations among loci?
- chromosome combinations rise or fall in frequency, by chance
how does mutation generate associations among loci?
- new mutations appear on particular genetic backgrounds
how does migration generate associations among loci?
- movement of individuals mixes different chromosomes
how does selection generate associations among loci?
- some chromosome combinations are fitter than others
evolution at multiple loci: what happens to allele frequencies when selection is absent? (2)
- all fitnesses = 1, so allele frequencies stay constant over time
- therefore, we know that multiple loci don’t cause allele frequencies to change without selection
evolution at multiple loci: what happens to chromosome frequencies when selection is absent (2)
- chromosome frequencies still chance as long as genetic associations (D) remain
- therefore, evolutionary change in chromosome types occurs because of the mixing from recombination, even without selection, drift, or mutation
evolution at multiple loci: what happens to genetic associations when selection is absent (2)
- genetic associations (D) decay by a factor of (1-r) because recombination breaks genetic associations apart
- therefore, genetic associations persist longer between nearby sites (that are separate by a low r), but even unlinked loci (r = 1/2) remain associated for a while
evolution at multiple loci: what happens if locus A is under selection and locus B is neutral? (2)
- dynamics at A are the same as before, but allele frequencies chance at locus B if D =/= 0
- therefore, selection at one site causes changes at other sites that are in disequilibrium with it
evolution at multiple loci: how does strong selection affect genetic hitchhiking
- stronger selection results in faster rise in frequency of a selected allele and more sites hitchhiking along (before being separated by recombination)
evolution at multiple loci: what happens if both sites are under selection (2)
- disequilibrium will be generated by selection if fitnesses interact (“epistasis”)
- therefore, selection builds up genetic associations (D) between alleles that “work well together”
evolution at multiple loci: what happens to mean fitness if both sites are under selection
- mean fitness can decline over time due to the combined effects of selection and recombination
- therefore, evolutionary change in a genome will not always increase adaptation and doesn’t “optimize” populations; rather, it works through the combined processes of selection, mutation, recombination, and drift
