R17 Genetic drift and natural selection

Question 1

What are the Hardy-Weinberg assumptions ?

Answer 1

• Diploid, sexual, non-overlapping generations
• Random mating (no assortative/disassortative mating)
• No inbreeding
• No mutation
• No selection
• No drift
• No migration

Question 2

If N=10, what proportion of the 10 populations do we expect the A1 allele to go to fixation after 100 generations?

Answer 2

0.5 (not random but always as close to 50% and 50% if the population is low)

Question 3

What is the value of p and q with no drift or selection ?

Answer 3

0.5 for both

Question 4

What is meant by allele fixation ?

Answer 4

fixation refers to the point when an allele (e.g., A1) has spread throughout a population to the point where it is the only allele present for a particular gene. At fixation, the allele frequency is 100%.

Question 5

What is meant by genetic drift ?

Answer 5

In a finite population, allele frequencies fluctuate randomly due to sampling effects each generation. Over time, this can lead to the fixation (or loss) of alleles purely by chance.

Question 6

What is assortative mating ?

Answer 6

Mating when individuals mate with others that are genotypically and phenotypically similar

Question 7

What is disassortative mating ?

Answer 7

Mating when individuals mate with others that are genotypically and phenotypically different

Question 8

What is the inbreeding coefficient and how do we calculate it ?

Answer 8

F = 1 - H.observed / 2pq

H.observed - observed frequency of heterozygotes
2pq - expected frequency of heterozygotes

larger number (up to 1) - more inbreeding

Question 9

What does genetic drift do to allele frequencies ?

Answer 9

• Change in allelic individuals
• Smaller population: larger change in allele frequency over generations, more generations would lead to allele fixation (the frequency stays constant or is lost)
• Smaller populations have a greater genetic drift

Question 10

What does selection do to allelic frequencies ?

Answer 10

• Heritable genetic differences in a population, these lead to different phenotypic differences, they all have different effects on survival and reproduction
• Selection can lead to fixation or loss of alleles (biased)

Question 11

What are the 3 types of selection?

Answer 11

• Directional selection: usually occurs in artificial selection or change in environment, one extreme is selected
• Disruptive selection: both extremes are selected and the normal is selected against
• Stabilising selection: the normal is selected for, both extremes are selected against

Question 12

How is relative fitness calculated?

Answer 12

the number of offspring/ highest number of potential offspring

closer to 1- more fit in the population

Question 13

How is selection coefficient ?

Answer 13

highest relative fitness – individuals fitness

Question 14

How does mutations affect allele frequencies ?

Answer 14

• Adds new alleles to a population (SNPs, INDELs, CNVs, structural variants)
• Starts at a frequency proportional to opulation size equal to 1/N (haploids) or ½ xN (diploids), N is the number of individuals

Question 15

How does migration affect allele frequencies

Answer 15

• Can add new alleles to a population
• Effect on frequency depends on how much migration and what alleles are in the source populations

Question 16

What is the 3 important factors of genetic drift ?

Answer 16

Genetic drift is down to random sampling (only some individuals reproduce and pass on their alleles), chance events (alleles may become more or less common, fitness is not taken into account), impact in small populations, these are more significant in the allele frequency in a small population

Question 17

What makes an allele go to fixation faster ?

Answer 17

• Selection pressure, positive is faster than negative
• Initial frequency being high
• Genetic drift – small population is faster
• Mutation rates being low – new alleles slow down fixation
• Migration – can speed up (if the same allele is brought in) or slows it down (if new alleles are brought in)
• Inbreeding – can speed up fixation
• Recombination – slows down fixation

Question 18

How does the frequency of a selected allele change ?

Answer 18

Increase

Question 19

How does drift affect allelic frequencies ?

Answer 19

random fluctation around the 0.5 mark for each allele frequency

Question 20

What does a large selection coefficient show ?

Answer 20

Large selective pressure against a particular genotype

reduced fitness

Question 21

Which ONE of the following statements is INCORRECT? Inbreeding:

1. occurs with the mating of related individuals in a population.
2. increases with reduction in population size.
3. can be associated with lower fitness of offspring.
4. increases the frequency of heterozygotes in the population.
5. can result from self-fertilization.

Answer 21

4

inbreeding decreases the frequency of heterozygotes

Question 22

Which ONE of the following statements is INCORRECT?

1. The founder effect alters allele frequencies.
2. The founder effect is associated with small populations.
3. The founder effect is the selection of a small proportion of the population because of its greater fitness.
4. The founder effect describes allele frequency changes arising by chance.
5. Because of the founder effect, rare mutant alleles sometimes reach a high frequency in some populations.

Answer 22

3

Question 23

Which ONE of the following definitions is INCORRECT?

1. Allele - One of a number of different forms of a gene at one specific locus.
2. Phenotype - The observable characteristics of an organism.
3. Homozygote - An organism with two copies of the same allele at a particular genetic locus
4. Dominant - An allele which, in a heterozygote, cannot be detected in combination with another allele.
5. Fitness - The proportion of reproducing descendants left by one genotype relative to the most successful genotype.

Answer 23

4

Dominant - an allele that can be detected in a heterozygote, as it masks the effect of the recessive allele when they are combined.

Question 24

If the fitnesses of three genotypes at a locus are:

       genotype                               aa       ab       bb

       fitness                                    1         1         0.8

which ONE of the following statements is INCORRECT?

1. The b allele is selected against.
2. The b allele is a deleterious recessive.
3. The selection coefficient acting on bb is 0.2.
4. If the fitnesses shown continue, the frequency of b will decline.
5. The locus shows heterozygous disadvantage.

Answer 24

5

Question 25

Which ONE of the following is NOT an assumption of the Hardy-Weinberg law?

1. The organism is diploid.
2. The population is small.
3. Gametes unite at random.
4. Natural selection is not operating on the locus.
5. The organism is sexual.

Answer 25

2

Question 26

What does a large inbreeding coefficient (F) show ?

Answer 26

High levels of inbreeding, increased homozygosity, reduced genetic diversity, accumulation of deleterious recessive alleles, indicates a small population