Population Genetics 6

Question 1

why don’t we see plants that have equal lengths for both male and female parts?

Answer 1

• to avoid selfing/inbreeding

Question 2

are sites experiencing selection dependent or independent, why? (2)

Answer 2

• they are dependent

- embedded within genes, within chromosomes, and within genomes

Question 3

locus

Answer 3

• site or location on a strand of DNA

Question 4

in the absence of genetic associations, how would we model the frequency of chromosome type?

Answer 4

• it would be the product of the component allele frequencies (frequency of allele A x frequency of allele B = frequency of AB chromosome)

Question 5

disequilibrium (3)

• symbol
• aka
• definition

Answer 5

• symbol: D
• aka linkage disequilibrium
• measures genetic associations among two loci and indicates which alleles tend to be found together at two loci

Question 6

what do the values of D tell us (4)

Answer 6

• 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

Question 7

recombination

Answer 7

• occurs during meiosis in sexual organisms to generate gametes carrying new combinations of alleles

Question 8

recombination rate (2)

• symbol
• definition

Answer 8

• symbol: r
• determines the probability of a cross-over event that creates non-parental gametes between two loci; a measure of genetic distance

Question 9

what is the formula for predicting the rate at which parental chromosomes are produced

Answer 9

1 - r; (1-r)/n for each of the parental combinations possible

Question 10

what is the formula for predicting the rate at which parental chromosomes are not produced

Answer 10

r; r/n for each non-parental combination possible

Question 11

why do we expect linkage disequilibrium to be found in genomes (4)

Answer 11

• because several processes generate associations along loci: drift, mutation, migration, selection

Question 12

how does drift generate associations among loci?

Answer 12

• chromosome combinations rise or fall in frequency, by chance

Question 13

how does mutation generate associations among loci?

Answer 13

• new mutations appear on particular genetic backgrounds

Question 14

how does migration generate associations among loci?

Answer 14

• movement of individuals mixes different chromosomes

Question 15

how does selection generate associations among loci?

Answer 15

• some chromosome combinations are fitter than others

Question 16

evolution at multiple loci: what happens to allele frequencies when selection is absent? (2)

Answer 16

• 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

Question 17

evolution at multiple loci: what happens to chromosome frequencies when selection is absent (2)

Answer 17

• 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

Question 18

evolution at multiple loci: what happens to genetic associations when selection is absent (2)

Answer 18

• 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

Question 19

evolution at multiple loci: what happens if locus A is under selection and locus B is neutral? (2)

Answer 19

• 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

Question 20

evolution at multiple loci: how does strong selection affect genetic hitchhiking

Answer 20

• stronger selection results in faster rise in frequency of a selected allele and more sites hitchhiking along (before being separated by recombination)

Question 21

evolution at multiple loci: what happens if both sites are under selection (2)

Answer 21

• disequilibrium will be generated by selection if fitnesses interact (“epistasis”)
• therefore, selection builds up genetic associations (D) between alleles that “work well together”

Question 22

evolution at multiple loci: what happens to mean fitness if both sites are under selection

Answer 22

• 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