Chapter 7 Need to Study Flashcards
What is heterozygote advantage (overdominance)?
It occurs when heterozygous individuals have higher fitness than either homozygote, maintaining both alleles in the population.
What is the equation for equilibrium allele frequency under overdominance?
p̂ = t / (s + t)
where s and t are selection coefficients for each homozygous genotype.
What is frequency-dependent selection?
A type of balancing selection where the fitness of a phenotype depends on its frequency in the population.
What is mutation-selection balance?
It is the equilibrium where the rate of new mutations equals the rate at which selection removes them from the population.
What is the equation for mutation-selection balance?
q̂ = √(μ / s)
where μ is the mutation rate and s is the selection coefficient.
Why might cystic fibrosis have a higher allele frequency than predicted by mutation-selection balance?
It may also be influenced by overdominance, as carriers might have resistance to typhoid fever.
What is assortative mating?
When individuals with similar phenotypes or genotypes mate more frequently than expected under random mating.
What is disassortative mating?
When individuals prefer mates with different phenotypes or genotypes, increasing heterozygosity.
What is inbreeding depression?
A reduction in fitness due to increased expression of deleterious recessive alleles in homozygous individuals.
What is the inbreeding coefficient (F)?
The probability that two alleles are identical by descent.
What is an example of inbreeding effects in history?
The Spanish Habsburg dynasty suffered from genetic disorders and infertility due to high inbreeding.
What are some inbreeding avoidance mechanisms?
- Disassortative mating (preference for different genotypes).
- Avoidance of mating with close relatives.
- Human preference for different MHC genotypes, potentially influencing mate choice through scent.
What is heterozygote advantage (overdominance)?
It occurs when heterozygous individuals have higher fitness than either homozygote, maintaining both alleles in the population.
How does overdominance affect allele frequencies?
It leads to an equilibrium frequency where both alleles are maintained in the population.
What is the equation for equilibrium allele frequency under overdominance?
𝑝^ = 𝑡 / (𝑠 + 𝑡)
where 𝑠 and 𝑡 are selection coefficients for each homozygous genotype.
Give an example of a condition maintained by overdominance.
Sickle cell trait, where heterozygotes (carriers) have resistance to malaria.
What is frequency-dependent selection?
A type of balancing selection where the fitness of a phenotype depends on its frequency in the population.
How does frequency-dependent selection maintain genetic variation?
When a phenotype becomes too common, its fitness decreases, favoring the rarer phenotype and maintaining both alleles in the population.
Give an example of frequency-dependent selection in nature.
Scale-eating cichlid fish: ‘left-mouthed’ and ‘right-mouthed’ forms alternate in frequency as prey adapt to defend against the more common type.
Why does mutation alone not drive rapid evolution?
Mutation rates are typically very low, and selection usually acts against most new mutations.
What is mutation-selection balance?
It is the equilibrium where the rate of new mutations equals the rate at which selection removes them from the population.
What is the equation for equilibrium allele frequency under mutation-selection balance?
𝑞^ = 𝜇 / 𝑠
where 𝜇 is the mutation rate and 𝑠 is the selection coefficient.
How does mutation-selection balance explain the persistence of some genetic disorders?
Even though selection removes harmful alleles, new mutations continually introduce them.
Why might cystic fibrosis have a higher allele frequency than predicted by mutation-selection balance?
It may also be influenced by overdominance, as carriers might have resistance to typhoid fever.
What is assortative mating?
When individuals with similar phenotypes or genotypes mate more frequently than expected under random mating.
What is disassortative mating?
When individuals prefer mates with different phenotypes or genotypes, increasing heterozygosity.
What is selfing (self-fertilization), and how does it affect genotype frequencies?
Selfing is extreme assortative mating where an organism mates with itself, increasing homozygosity over generations.
What is inbreeding depression?
A reduction in fitness due to increased expression of deleterious recessive alleles in homozygous individuals.
How can we measure inbreeding?
Using the inbreeding coefficient (𝐹), which represents the probability that two alleles are identical by descent.
What is the equation for genotype frequencies under inbreeding?
Homozygotes: 𝑝^2(1−𝐹) + 𝑝𝐹 and 𝑞^2(1−𝐹) + 𝑞𝐹
Heterozygotes: 2𝑝𝑞(1−𝐹)
where 𝑝 and 𝑞 are allele frequencies.
What are some real-world examples of inbreeding effects?
Spanish Habsburg dynasty: High inbreeding led to genetic disorders and infertility. European royalty: Hemophilia was common due to recessive allele exposure.
What are some mechanisms species use to avoid inbreeding?
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.
How does selection affect allele frequencies?
Beneficial alleles increase in frequency, while harmful alleles decrease, depending on their dominance and selection strength.
What happens when selection acts against a common recessive allele?
Evolution occurs rapidly because many individuals express the recessive phenotype.
What happens when selection acts against a rare recessive allele?
Evolution is slow because the allele is mostly hidden in heterozygotes.
Why do recessive alleles persist in a population despite being selected against?
They can ‘hide’ in heterozygous individuals, where they are not subject to selection.
How is the strength of selection (s) calculated?
s = 1 - (fitness of selected genotype).
If homozygous recessive individuals have 40% mortality, what is the selection coefficient (s)?
s = 0.4 (since 60% survive).
If heterozygotes have 55% survival compared to homozygotes, what is the selection coefficient?
s = 1 - 0.55 = 0.45.
How does coat color in rock pocket mice relate to selection?
Dark mice are favored on lava flows, while light mice are favored on desert sand, leading to selection based on habitat.
In rock pocket mice, if the light-colored genotype (A2A2) has a fitness of 0.9, what is the selection coefficient (s)?
s = 1 - 0.9 = 0.1.
What equation predicts the frequency of a recessive allele after selection?
q’ = q(1 - sq) / (1 - sq^2)
where q’ is the new frequency, q is the current frequency, and s is the selection coefficient.
If a recessive allele starts at q = 0.2 and selection coefficient s = 0.1, what will q’ be in the next generation?
q’ = 0.2(1 - (0.2 × 0.1)) / (1 - (0.1 × 0.2^2))
q’ ≈ 0.196 (showing a slight decrease in allele frequency due to selection).
Why are most common genetic diseases recessive?
Because recessive alleles can persist in heterozygous carriers, avoiding selection.
Why are dominant disadvantageous alleles rare?
They are always expressed and selected against unless they appear late in life (e.g., Huntington’s disease).
Give an example of an autosomal recessive disease and its prevalence.
Cystic fibrosis (1 in 2,000 Caucasians).
Give an example of an autosomal dominant disease and its prevalence.
Huntington’s disease (1 in 2,500).
What are the three main types of selection?
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.
Which type of selection can completely eliminate a recessive allele?
Strong directional selection.
