Epistasis and selection

Question 1

What is epistasis?

Answer 1

Interaction between genes where 1 gene affects or masks expression of another gene

Question 2

When does epistasis specifically occur?

Answer 2

Genes on different chromosomes interact influencing same phenotype leading to unexpected phenotypic ratios from crosses

Question 3

What is a hypostatic gene?

Answer 3

Gene whose expression is blocked by another gene

Question 4

What is an epistatic gene?

Answer 4

Gene whose allele affects expression of hypostatic gene

Question 5

What are the 2 types of epistasis?

Answer 5

• dominant
• recessive

Question 6

What is recessive epistasis?

Answer 6

When epistatic gene must be homozygous recessive to block expression of hypostatic gene

Question 6

What is dominant epistasis?

Answer 6

When epistatic gene is dominant actively modifying or blocking expression of hypostatic gene

Question 7

What is evolution?

Answer 7

Process where inherited characteristics of a population change over generations

Question 8

Why does evolution occur?

Answer 8

Result of variations in frequency of different alleles within population’s gene pool

Question 9

What are several key factors affecting evolution by altering allele frequency?

Answer 9

Mutation = introduce new alleles and genetic variation

Gene flow = transfers alleles between populations

Genetic drift = causes random changes in allele frequencies impacting small populations more significantly

Natural selection = enhances frequency of alleles improving survival and reproductive success

Sexual selection = increases frequency of alleles enhancing reproductive success specifically

Question 10

What is genetic drift?

Answer 10

Random changes in allele frequency within population’s gene pool due to chance events

Question 11

How does a large population impact genetic diversity?

Answer 11

Larger gene pool means genetic diversity is high

can adapt effectively to changing selection pressures through natural selection

Question 12

How does a small population impact genetic diversity?

Answer 12

Smaller gene pool means genetic diversity is low

at risk of extinction when there are changes in selection pressures

Question 13

What 2 factors can limit population size?

Answer 13

Density-dependent factors = depend on population size e.g competition, predation and disease

Density-independent factors = impact populations regardless of size e.g natural disasters and climate change

Question 14

What is the bottleneck effect?

Answer 14

Occurs when population’s size reduces suddenly and drastically and reduction lasts for at least 1 generation

Question 15

What are some implications of population bottlenecks?

Answer 15

• reduced gene pool and decreased genetic diversity compared to original population
• decreased genetic diversity causes issues related to inbreeding and reduced fertility
• may also allow a beneficial mutation to become more prevalent

Question 16

What is the founder effect?

Answer 16

Small group splits from larger population and small new population established by small number of individuals

Question 17

What are potential impacts of the founder effect?

Answer 17

• reduced gene pool and decreased genetic diversity compared to original population
• rare alleles from original population may become more common in new population

Question 18

How can variation drive evolution?

Answer 18

• generates range of phenotypes within population enhancing likelihood some individuals have alleles for advantageous traits
• individuals with beneficial traits more likely to survive and reproduce under changing conditions, transmitting advantageous alleles to offspring
• natural selection occurs

Question 19

What is directional selection?

Answer 19

Selects for 1 extreme phenotype over other phenotypes

Question 20

What is stabilising selection?

Answer 20

Selects for average phenotype and selects against extreme phenotypes

Question 21

What is disruptive selection?

Answer 21

Selects for extreme phenotypes selecting against intermediate phenotype especially when an environmental factor takes 2 or more distinct forms

Question 22

What are the affects of directional selection and give an example?

Answer 22

• increased allele frequency for 1 extreme phenotype
• shifts curve in direction of favoured extreme

e.g antibiotic resistance in bacteria

Question 23

What are the affects of stabilising selection and give an example?

Answer 23

• increased allele frequency for average phenotype but decreased allele frequency for extremes
• narrows curve

e.g human birth weights

Question 24

What are the affects of disruptive selection and give an example?

Answer 24

- increased allele frequency for multiple extreme phenotypes but decreased allele frequency for intermediates - curve shifts into multiple peaks either side of where average phenotype peak was e.g bird beaks adapting to become larger and smaller when there are 2 different food sources

Question 25

What is the Hardy-Weinberg principle?

Answer 25

Mathematical equation helping calculate frequency of alleles for a particular gene within a population

Question 26

What are key assumptions in the Hardy-Weinberg principle?

Answer 26

- no mutations occur - no migration in or out of population - mating is random - population size is large - no natural selection pressures

Question 27

What is the Hardy-Weinberg equation?

Answer 27

p² + 2pq + q² = 1

Question 28

What does each part of the Hardy-Weinberg equation mean?

Answer 28

p = frequency of dominant allele q = frequency of recessive allele p2 = frequency of homozygous dominant individuals 2pq = frequency of heterozygous individuals q2 = frequency of homozygous recessive individuals