Hard Stuff on Bio 1780

Question 1

• What does Heterozygosity depend on? How do they each affect the degree of heterozygosity?

Answer 1

Mutation & population size. Increase in both mutation rate and population size will increase heterozygosity

Question 2

genetic drift

Answer 2

change in allele frequency over time due to random chance

Question 3

how drift affects small populations

Answer 3

higher probability of large jumps in allele frequency any given generation; shorter time to fixation or loss

Question 4

probability of brand new mutation becoming fixed

Answer 4

1/ (2N)

Question 5

relationship between drift and mutation

Answer 5

H = (4Nu)/(1+4Nu)

Question 6

When N gets larger, H ….

Answer 6

increases

Question 7

When u gets larger, H…

Answer 7

increases

Question 8

Define the Bottleneck effect and predict its outcome

Answer 8

Sudden external factor (e.g. natural disaster) that kills most of the individuals in a population, only leaving a
randomly selected small group of individuals. Genetic drift will cause it to evolve from the previous genotypic
frequencies

Question 9

Selection happens when individuals

Answer 9

vary in fitness, due to some having higher rates of survival and/or reproduction

Question 10

Modes of natural selection

Answer 10

1. Purifying selection
2. Directional selection
3. Disruptive selection (favors extreme
   phenotypes)
4. Overdominance (heterozygote advantage)
5. Underdominance (heterozygote disadvantage)
6. Frequency dependent selection

Question 11

Common modes of natural selection

Answer 11

purifying and directional (selection acts on phenotypes regardless of genetic basis)

Question 12

Why isn’t purifying selection perfectly effective at removing deleterious alleles

Answer 12

o Not all phenotypic variation has a simple genetic basis
o Deleterious recessive alleles can be sheltered in heterozygotes
 Because paired with other allele that is in some way more beneficial
o Mutation continually introduces genetic variation into population

Question 13

If new mutation is codominant and advantageous

Answer 13

codominant genotype will be selected for at a slow rate, causing a gradual rise in f(B). However, once BB homozygotes are present in the population from the mating of ABxAB, there will be selection for BB even over the codominant case. Thus, allele B will eventually reach fixation, since genotype BB is being selected for above all else; will take more time before initial slope because evolution proceeds more slowly since advantage is small

Question 14

If new mutation is dominant and advantageous

Answer 14

like codominant but initial slope occurs before; may not become fixed or takes longer because A2 alleles will persist because sheltered in heterozygotes but selection will remove it

Question 15

If new mutation is recessive and advantageous

Answer 15

evolution is delayed by selection because everyone has same fitness; allele stays very rare and frequency determined by drift

Question 16

natural selection is strong…..

Answer 16

in large populations

Question 17

if N x s is less than 1 ….

Answer 17

drift is more powerful force, where N is population size and s is strength of selection on selected allele

Question 18

if N x s is greater than 1…

Answer 18

selection is more powerful

Question 19

Heritability (the proportion of trait variance that is genetic) gets small when

Answer 19

environmental contributions to the trait get large

Question 20

Heritability predicts how well population responds to

Answer 20

directional selections

Question 21

R

Answer 21

response to selection; difference between the trait mean in the progeny and the trait mean in parents

Question 22

large S means

Answer 22

strong selection