Population genetics

Question 1

What is population genetics

Answer 1

population genetics is the study of the genetic variation in a population, and the evolutionary factors that explain this variation

Question 2

What are evolutionary processes

Answer 2

evolution: a change in allele and/or genotype frequency over time

evolutionary processes included mutation and recombination, natural selection, gene flow, genetic drift and non-random mating

Question 3

What is the Hardy-Weinberg equilibrium

Answer 3

• describes situations in which allele and genotype frequencies do not change
• based on Mendelian genetics but at a population level

Question 4

What are the assumptions of the Hardy-Weinberg equilibrium

Answer 4

• no selection
• no migration
• no mutation
• large population size (no genetic drift)
• random mating occurs

no evolution occurs

Question 5

Example of Hardy-Weinberg equation
(need to use known genotype so normally recessive)

Answer 5

In the UK, 1 in 2500 babies are born with CF. What is the frequency of heterozygous carriers in the population

q squared = homozygous recessive with CF
1/2500
q = root of 1/2500 = 0.02

p + q =1
p = 1 - 0.02 = 0.98

p2 + 2pq + q2 = 1
2pq = 1 - 0.964 - 0.0004 = 0.0392

frequency of heterozygous carriers of CF is 0/0392

Question 6

What are mutations

Answer 6

mutations are needed in order for evolution to occur

• source of new alleles and genetic variation
• provides ‘raw material’ for other evolutionary forces to act on

Question 7

What is natural selection

Answer 7

variation in average reproductive success among phenotype

-based on overproduction in natural phenotypes
- individuals compete for resources, phenotypic variation occurs within the population and some phenotypes may have better chance of reproducing than others

• evolution occurs if phenotype with different reproductive success has a genetic basis

fitness: measure of the relative reproductive success of a genotype
adaptation: a consequence of selection - prevalent or maintained in a population because of natural selection for that function

Question 8

What are the different types of selection

Answer 8

-positive selection
- negative selection
- balancing selection

Question 9

What is positive selection

Answer 9

natural selection that increases the frequency of a favourite allele

Question 10

What is negative selection

Answer 10

natural selection that decreases the frequency of a deleterious allele

Question 11

What is balancing selection

Answer 11

natural selection that acts to maintain two or more alleles in a population
eg. sickle cell anaemia
heterozygote advantage: AA vulnerable to malaria, SS affected by sickle cell anaemia
- S and A alleles are maintained in the population at intermediate frequencies in regions of malaria

Question 12

What are the different types of selection that effect a particular trait

Answer 12

• stabilising selection
• directional selection
• disruptive selection

Question 13

What is stabilising selection

Answer 13

maintain the status quo and acts against extremes
eg. babies size at birth

Question 14

What is directional selection

Answer 14

directional selection leads to a change in trait over time

eg. a drought occurred and only plants with large seeds survived
- birds with a bigger beak were then more likely to survive and the population changed so birds had a bigger beak size

artificial selection is where humans choose a trait which drives selection

Question 15

What is disruptive selection

Answer 15

operates in favour of extremes and against intermediate forms
eg. evolution of two distinct populations of apple
- maggot flies in North America after apples were introduced from Europe

• this can lead to the formation of new species

Question 16

What is sexual selection

Answer 16

sexual selection promotes traits that increase an individuals access to reproductive opportunities

intersexual selection: members of one sex compete for the attention of another sex
intrasexual selection: members of one sex compete for access to the other sex

Question 17

What is gene flow

Answer 17

• the movement of alleles from one population to another (caused by migration)
  -increases genetic diversity in the receiving population but makes populations more similar to each other
• can be maladaptive

Question 18

What is genetic drift

Answer 18

random changes in allele frequency from generation to generation (effect is stronger on smaller populations)

• in extreme cases a population bottleneck occur where there is a catastrophic reduction in population, this causes a loss of genetic variation

founder effect: a few individuals of a population starts a new population
- leads to a loss of variation

Question 19

What is non-random mating

Answer 19

assortative mating: individual with the SAME genome more likely to mate than individuals with different genotypes

disassortative mating: individuals more likely to mate with individuals of a DIFFERENT genotype

non-random mating leads to a change of genotype frequencies, but not a change in allele frequencies

assortative mating: excess of homozygotes (inbreeding)

disassortative mating: excess of heterozygotes