Chapter 7 Need to Study

Question 2

What is heterozygote advantage (overdominance)?

Answer 2

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

Question 3

What is the equation for equilibrium allele frequency under overdominance?

Answer 3

p̂ = t / (s + t)

where s and t are selection coefficients for each homozygous genotype.

Question 4

What is frequency-dependent selection?

Answer 4

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

Question 5

What is mutation-selection balance?

Answer 5

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

Question 6

What is the equation for mutation-selection balance?

Answer 6

q̂ = √(μ / s)

where μ is the mutation rate and s is the selection coefficient.

Question 7

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

Answer 7

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

Question 8

What is assortative mating?

Answer 8

When individuals with similar phenotypes or genotypes mate more frequently than expected under random mating.

Question 9

What is disassortative mating?

Answer 9

When individuals prefer mates with different phenotypes or genotypes, increasing heterozygosity.

Question 10

What is inbreeding depression?

Answer 10

A reduction in fitness due to increased expression of deleterious recessive alleles in homozygous individuals.

Question 11

What is the inbreeding coefficient (F)?

Answer 11

The probability that two alleles are identical by descent.

Question 12

What is an example of inbreeding effects in history?

Answer 12

The Spanish Habsburg dynasty suffered from genetic disorders and infertility due to high inbreeding.

Question 13

What are some inbreeding avoidance mechanisms?

Answer 13

• Disassortative mating (preference for different genotypes).
• Avoidance of mating with close relatives.
• Human preference for different MHC genotypes, potentially influencing mate choice through scent.

Question 15

What is heterozygote advantage (overdominance)?

Answer 15

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

Question 16

How does overdominance affect allele frequencies?

Answer 16

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

Question 17

What is the equation for equilibrium allele frequency under overdominance?

Answer 17

𝑝^ = 𝑡 / (𝑠 + 𝑡)

where 𝑠 and 𝑡 are selection coefficients for each homozygous genotype.

Question 18

Give an example of a condition maintained by overdominance.

Answer 18

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

Question 19

What is frequency-dependent selection?

Answer 19

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

Question 20

How does frequency-dependent selection maintain genetic variation?

Answer 20

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

Question 21

Give an example of frequency-dependent selection in nature.

Answer 21

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

Question 22

Why does mutation alone not drive rapid evolution?

Answer 22

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

Question 23

What is mutation-selection balance?

Answer 23

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

Question 24

What is the equation for equilibrium allele frequency under mutation-selection balance?

Answer 24

𝑞^ = 𝜇 / 𝑠

where 𝜇 is the mutation rate and 𝑠 is the selection coefficient.

Question 25

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

Answer 25

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

Question 26

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

Answer 26

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

Question 27

What is assortative mating?

Answer 27

When individuals with similar phenotypes or genotypes mate more frequently than expected under random mating.

Question 28

What is disassortative mating?

Answer 28

When individuals prefer mates with different phenotypes or genotypes, increasing heterozygosity.

Question 29

What is selfing (self-fertilization), and how does it affect genotype frequencies?

Answer 29

Selfing is extreme assortative mating where an organism mates with itself, increasing homozygosity over generations.

Question 30

What is inbreeding depression?

Answer 30

A reduction in fitness due to increased expression of deleterious recessive alleles in homozygous individuals.

Question 31

How can we measure inbreeding?

Answer 31

Using the inbreeding coefficient (𝐹), which represents the probability that two alleles are identical by descent.

Question 32

What is the equation for genotype frequencies under inbreeding?

Answer 32

Homozygotes: 𝑝^2(1−𝐹) + 𝑝𝐹 and 𝑞^2(1−𝐹) + 𝑞𝐹 Heterozygotes: 2𝑝𝑞(1−𝐹) ## Footnote where 𝑝 and 𝑞 are allele frequencies.

Question 33

What are some real-world examples of inbreeding effects?

Answer 33

Spanish Habsburg dynasty: High inbreeding led to genetic disorders and infertility. European royalty: Hemophilia was common due to recessive allele exposure.

Question 34

What are some mechanisms species use to avoid inbreeding?

Answer 34

Disassortative mating (preference for different genotypes). Avoidance of mating with close relatives. Human preference for different MHC (immune system) genotypes, potentially influencing mate choice through scent.

Question 35

How does selection affect allele frequencies?

Answer 35

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

Question 36

What happens when selection acts against a common recessive allele?

Answer 36

Evolution occurs rapidly because many individuals express the recessive phenotype.

Question 37

What happens when selection acts against a rare recessive allele?

Answer 37

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

Question 38

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

Answer 38

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

Question 39

How is the strength of selection (s) calculated?

Answer 39

s = 1 - (fitness of selected genotype).

Question 40

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

Answer 40

s = 0.4 (since 60% survive).

Question 41

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

Answer 41

s = 1 - 0.55 = 0.45.

Question 42

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

Answer 42

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

Question 43

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

Answer 43

s = 1 - 0.9 = 0.1.

Question 44

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

Answer 44

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 45

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

Answer 45

q' = 0.2(1 - (0.2 × 0.1)) / (1 - (0.1 × 0.2^2)) ## Footnote q' ≈ 0.196 (showing a slight decrease in allele frequency due to selection).

Question 46

Why are most common genetic diseases recessive?

Answer 46

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

Question 47

Why are dominant disadvantageous alleles rare?

Answer 47

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

Question 48

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

Answer 48

Cystic fibrosis (1 in 2,000 Caucasians).

Question 49

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

Answer 49

Huntington’s disease (1 in 2,500).

Question 50

What are the three main types of selection?

Answer 50

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 51

Which type of selection can completely eliminate a recessive allele?

Answer 51

Strong directional selection.