Chapter 7

Question 1

What does population genetics integrate?

Answer 1

It integrates evolution by natural selection with Mendelian genetics.

Question 2

What is microevolution?

Answer 2

It is the change in genotype frequencies over time within a population.

Question 3

What is the Hardy-Weinberg principle?

Answer 3

It is a null model that predicts genotype frequencies if no evolution is occurring.

Question 4

What five main evolutionary processes disrupt Hardy-Weinberg equilibrium?

Answer 4

Selection, mutation, migration, genetic drift (finite population size), and non-random mating.

Question 5

What happens when a population is in Hardy-Weinberg equilibrium?

Answer 5

Allele frequencies remain constant from generation to generation.

Question 6

How do you calculate allele frequencies from genotype frequencies?

Answer 6

Freq(A) = Freq(AA) + ½ Freq(Aa)
Freq(a) = Freq(aa) + ½ Freq(Aa)

Question 7

Given a population where the genotype frequencies are AA = 0.25, Aa = 0.50, and aa = 0.25, what are the allele frequencies?

Answer 7

Freq(A) = 0.25 + (0.5 × 0.50) = 0.50
Freq(a) = 0.25 + (0.5 × 0.50) = 0.50

Question 8

What is the Hardy-Weinberg equation?

Answer 8

p² + 2pq + q² = 1, where:
p² = Frequency of homozygous dominant (AA)
2pq = Frequency of heterozygotes (Aa)
q² = Frequency of homozygous recessive (aa)

Question 9

If a population is not in Hardy-Weinberg equilibrium, what does that indicate?

Answer 9

It suggests that one or more evolutionary forces are acting on the population.

Question 10

In a study of a mosquito population, 40% were homozygous resistant (rr), 50% were heterozygous (rR), and 10% were homozygous susceptible (RR). What is the frequency of the resistant allele (r)?

Answer 10

Freq(r) = (1 × 0.40) + (0.5 × 0.50) = 0.40 + 0.25 = 0.65

Question 11

If an allele (D) that decreases pesticide penetration has a frequency of 0.8 in a population, what are the expected genotype frequencies under Hardy-Weinberg equilibrium?

Answer 11

Freq(DD) = 0.8² = 0.64
Freq(Dd) = 2(0.8)(0.2) = 0.32
Freq(dd) = 0.2² = 0.04

Question 12

What can cause deviations from Hardy-Weinberg equilibrium?

Answer 12

Natural selection, genetic drift, mutation, migration, and non-random mating.

Question 13

In a deer population, the allele for shorter antlers (a) has a frequency of 0.23. The measured genotypic frequencies differ from Hardy-Weinberg predictions. What does this suggest?

Answer 13

Evolution is occurring in the population.

Question 14

What is an example of a real-world application of Hardy-Weinberg principles?

Answer 14

Studying how malaria resistance alleles are maintained in human populations.

Question 15

In a study on malaria resistance, observed genotype frequencies did not match Hardy-Weinberg predictions. What does this suggest?

Answer 15

Selection is occurring, possibly due to heterozygote advantage.

Question 16

What is heterozygote advantage?

Answer 16

It is when heterozygous individuals have higher fitness than either homozygous genotype, maintaining both alleles in the population.

Question 17

What is the main assumption of Hardy-Weinberg equilibrium that selection violates?

Answer 17

The assumption of no selection.

Question 18

Why can’t we use Hardy-Weinberg equilibrium to predict genotype frequencies when selection is occurring?

Answer 18

Because one of the key assumptions (no selection) is violated, altering allele frequencies over generations.

Question 19

How does selection affect allele frequencies?

Answer 19

Beneficial alleles increase in frequency, while harmful alleles decrease, depending on their dominance and selection strength.

Question 20

What happens when selection acts against a common recessive allele?

Answer 20

Evolution occurs rapidly because many individuals express the recessive phenotype.

Question 21

What happens when selection acts against a rare recessive allele?

Answer 21

Evolution is slow because the allele is mostly hidden in heterozygotes.

Question 22

Why do recessive alleles persist in a population despite being selected against?

Answer 22

They can ‘hide’ in heterozygous individuals, where they are not subject to selection.

Question 23

How is the strength of selection (s) calculated?

Answer 23

s = 1 - (fitness of selected genotype).

Question 24

If homozygous recessive individuals have 40% mortality, what is the selection coefficient (s)?

Answer 24

s = 0.4 (since 60% survive).

Question 25

If heterozygotes have 55% survival compared to homozygotes, what is the selection coefficient?

Answer 25

s = 1 - 0.55 = 0.45.

Question 26

How does coat color in rock pocket mice relate to selection?

Answer 26

Dark mice are favored on lava flows, while light mice are favored on desert sand, leading to selection based on habitat.

Question 27

In rock pocket mice, if the light-colored genotype (A2A2) has a fitness of 0.9, what is the selection coefficient (s)?

Answer 27

s = 1 - 0.9 = 0.1.

Question 28

What equation predicts the frequency of a recessive allele after selection?

Answer 28

q' = q(1 - sq) / (1 - sq^2) ## Footnote where q' is the new frequency, q is the current frequency, and s is the selection coefficient.

Question 29

If a recessive allele starts at q = 0.2 and selection coefficient s = 0.1, what will q' be in the next generation?

Answer 29

q' ≈ 0.196 (showing a slight decrease in allele frequency due to selection).

Question 30

Why are most common genetic diseases recessive?

Answer 30

Because recessive alleles can persist in heterozygous carriers, avoiding selection.

Question 31

Why are dominant disadvantageous alleles rare?

Answer 31

They are always expressed and selected against unless they appear late in life (e.g., Huntington’s disease).

Question 32

Give an example of an autosomal recessive disease and its prevalence.

Answer 32

Cystic fibrosis (1 in 2,000 Caucasians).

Question 33

Give an example of an autosomal dominant disease and its prevalence.

Answer 33

Huntington’s disease (1 in 2,500).

Question 34

What are the three main types of selection?

Answer 34

Directional selection: Shifts the mean trait value in one direction. Stabilizing selection: Reduces trait variation but maintains the mean. Disruptive selection: Increases trait variation without changing the mean.

Question 35

Which type of selection can completely eliminate a recessive allele?

Answer 35

Strong directional selection.

Question 36

What is heterozygote advantage (overdominance)?

Answer 36

It occurs when heterozygous individuals have higher fitness than either homozygote, maintaining both alleles in the population.

Question 37

How does overdominance affect allele frequencies?

Answer 37

It leads to an equilibrium frequency where both alleles are maintained in the population.

Question 38

What is the equation for equilibrium allele frequency under overdominance?

Answer 38

p(hat) = t / (s+t)

Question 39

Give an example of a condition maintained by overdominance.

Answer 39

Sickle cell trait, where heterozygotes (carriers) have resistance to malaria.

Question 40

What is frequency dependent selection?

Answer 40

A type of balancing selection where the fitness of a phenotype depends on its frequency in the population.

Question 41

How does frequency-dependent selection maintain genetic variation?

Answer 41

When a phenotype becomes too common, its fitness decreases, favoring the rarer phenotype and maintaining both alleles in the population.

Question 42

Give an example of frequency-dependent selection in nature.

Answer 42

Scale-eating cichlid fish: "left-mouthed" and "right-mouthed" forms alternate in frequency as prey adapt to defend against the more common type.

Question 43

Why does mutation alone not drive rapid evolution?

Answer 43

Mutation rates are typically very low, and selection usually acts against most new mutations.

Question 44

What is mutation-selection balance?

Answer 44

The equilibrium where the rate of new mutations equals the rate at which selection removes them from the population.

Question 45

What is the equation for eq. frequency under mutation-selection balance?

Answer 45

q(hat) = sq. rt. (mu/S)

Question 46

How does mutation-selection balance explain the persistence of some genetic disorders?

Answer 46

Even though selection removes harmful alleles, new mutations continually introduce them.

Question 47

Why might cystic fibrosis have a higher allele frequency than predicted by mutation-selection balance?

Answer 47

It may also be influenced by overdominance, as carriers might have resistance to typhoid fever.