Population Genetics 5

Question 1

genetic drift

Answer 1

• change in allele frequencies that results from random sampling processes that take place within populations over generations

Question 2

when do sampling events take place within populations (4)

Answer 2

• sample of adults contribute to gamete pool
• sample of gametes combine to make zygotes
• same of zygotes survive to become juveniles
• sample of juveniles survive to become adults

Question 3

when does drift act

Answer 3

• all the time because all populations are finite

Question 4

genetic drift and population size (2)

Answer 4

• changes in allele frequencies from one generation to the next are expected to be greater in smaller populations
• populations remain polymorphic for longer, on average, with increasing population size; larger populations are better able to retain genetic variation

Question 5

how does drift act on different loci across a genome (2)

Answer 5

• with population size being equal, drift acts similarly across all loci in the genome
• it will affect all loci carried by an individual, meaning it acts on good, bad and neutral alleles

Question 6

what is the probability that a neutral allele will fix due to genetic drift

Answer 6

• the change that a neutral allele is the “lucky” one that happens to fix is given by its initial frequency

Question 7

coalescence time

Answer 7

• the “backwards” time it would take for all alleles to descend from the same single ancestral allele

Question 8

how is genetic drift affected by the number of alleles within a population

Answer 8

on average, the amount of time it takes for a single allele to fix is twice the number of alleles in the population

• 2N generations with N haploid individuals
• 4N generations with N diploid individuals (2N alleles)

Question 9

expected heterozygosity

Answer 9

H[t] = 2p[t]q[t]

- probability that two alleles drawn at random are different alleles

Question 10

what are the two possibilities that can occur when we sample two random alleles

Answer 10

• sampled alleles descended from the same parent allele

- sampled alleles descend from different parent alleles

Question 11

what is the probability that sampled alleles descended from the same parent allele when randomly sampling two alleles in a population

Answer 11

1/2N

Question 12

what is the probability that sampled alleles descended from different parent alleles when randomly sampling two alleles in a population

Answer 12

1 - (1/2N)

Question 13

how does expected heterozygosity change over time due to drift (2)

Answer 13

• genetic variability declines by 1/(2N) every generation in a population of N diploids
• loss of heterozygosity is more rapid in smaller populations

Question 14

why might heterozygosity drop faster than expected (2)

Answer 14

• selective mating (unequal changes of all parents producing offspring; less variation into the next generation)
• inbreeding depression

Question 15

expected homozygosity (4)

Answer 15

F[t] = 1 - H[t]

• probability that two alleles drawn at random are the same allele
• describes increase in homozygosity/inbreeding when alleles have recently descended from the same ancestor in a finite population (“inbreeding”)
• small populations become “inbred” with little genetic variation remaining due to drift

Question 16

founder effect (2)

Answer 16

• chance effects that occur when a new population is founded by a small number of individuals from a larger source population
• can cause high incidence of some disease alleles like it did in the descendants of French Canadian settlers

Question 17

genetic bottleneck

Answer 17

• a severe reduction in the number of individuals in a population, which results in the loss of genetic variation in the surviving population

Question 18

genetic drift and founder effect

Answer 18

• events where populations decrease rapidly in size, there is a loss of genetic variation, and genetic drift has a large impact

Question 19

inbreeding

Answer 19

• with few surviving lineages, the chance that two alleles in an individual recently shared a common ancestor is much higher (“inbreeding”), increasing the frequency of homozygous recessive diseases (“inbreeding depression”)