Cycle 7 - Bio 1001

What common misconception did the BBC article about blonde hair illustrate?

The misconception that recessive alleles automatically decrease in frequency over time and eventually disappear.

Does an allele's dominance status (dominant vs. recessive) directly affect its frequency in a population?

No. An allele's dominance status only determines how it is expressed in a phenotype, not how often it is passed down.

• If a population has a dominant allele (\(p = 0.6\)) and a recessive allele (\(q = 0.4\)), how will the allele frequencies change if there is no natural selection

They will not change. Without evolutionary forces like natural selection, allele frequencies remain constant from generation to generation (Hardy-Weinberg equilibrium).

Under what specific condition would a recessive allele actually decrease in frequency in a population?

• Only if there is natural selection acting against the phenotype (or other evolutionary forces like genetic drift or non-random mating).

If genotypes differ in fitness, does an allele's dominance status matter?

• Yes. When fitness varies, dominance status significantly affects how quickly allele frequencies change under natural selection.

Why is natural selection highly effective at removing a harmful dominant allele from a population?

• Because the dominant allele is expressed in both homozygotes and heterozygotes, exposing all carriers to selection immediately.

Why do harmful recessive alleles persist in a population and clear out much more slowly?

• When rare, most copies are hidden in healthy heterozygotes (carriers), shielding them from natural selection.

What is the fundamental difference between what "dominance" describes versus what "fitness" describes?

Dominance describes the relationship between genotype and phenotype. Fitness describes the relationship between genotype and environmental survival/reproduction.

What is the ultimate fate of a dominant allele if it is less fit for the environment?

• It will decrease in frequency until it completely disappears (reaches 0).

Why is a harmful recessive allele unable to fully die out from a population?

Because it hides in heterozygotes. Healthy carriers continue to pass the recessive allele down to future generations.

What is the stable equilibrium frequency for both alleles under a classic heterozygote advantage scenario?

• An allele frequency of 0.5. Both alleles will eventually balance out at this exact point.

Under heterozygote advantage, does the starting allele frequency affect the final equilibrium?

No. Regardless of where the initial allele frequencies start, they will always evolve toward and stabilize at the same 0.5 equilibrium point.

Is a population experiencing selection and evolution under heterozygote advantage?

Yes, it experiences selection because homozygotes are selected against. It evolves at first, but evolution stops once the allele frequencies stabilize at equilibrium.

What happens to allele frequencies when selection acts against heterozygotes?

One allele goes to fixation (frequency of 1) while the other completely dies out (frequency of 0).

Under homozygote advantage, why does the starting allele frequency matter?

Whichever allele starts off more common will reach fixation. The rare allele will be completely eliminated.

When does evolution happen under heterozygote advantage

Under heterozygote advantage, the population evolves at first, but not after allele frequencies stabilitze

Why do entire populations rarely show clean Mendelian ratios (like 3:1 or 1:2:1) seen in single families?

• Because in a single family, individual allele frequencies are restricted to 0, 0.5, or 1, whereas in a whole population, allele frequencies can be any continuous value from 0 to 1.

Conceptually, what baseline scenario does Hardy-Weinberg Equilibrium (HWE) represent?

HWE represents what a population looks like when it is NOT evolving at a specific genetic locus.

What are the core conditions/assumptions required for a population to remain in Hardy-Weinberg Equilibrium?

The population must be large, randomly mating, have no selection, no migration (gene flow), and negligible mutations.

Under HWE, if you know the allele frequencies, what can you directly predict?

You can directly predict the genotype frequencies of the population.

What are the mathematical formulas used to calculate genotype frequencies from allele frequencies (\(p\) and \(q\)) under HWE?

f(A₁A₁) = p²

^{f(A₁A₂) = 2pq}

f(A₂A₂) = q²

What does it mean if a population's observed genotype frequencies do not match the frequencies predicted by the HWE equations?

The population is not at HWE, meaning one or more assumptions have been violated and the population could be evolving at that locus.

"In a large, random-mating population with no mutations, migration, or selection, allele frequencies do not change and genotype frequencies..."

• ...can be predicted directly from allele frequencies (\(p^2, 2pq, q^2\)) and will remain constant from generation to generation.

What is the primary use of the Hardy-Weinberg principle in real-world evolutionary biology?

• It serves as a null model (baseline). By comparing observed population data against HWE expectations, scientists can detect if evolution is occurring and investigate which forces are causing it.

Under what phenotypic condition can heterozygote advantage or disadvantage occur?

Only when heterozygotes have a different phenotype from either homozygote (incomplete dominance or codominance).

Why can't heterozygote advantage or disadvantage occur under complete dominance?

Because heterozygotes would look identical to dominant homozygotes, preventing selection from treating them differently.

Where are rare alleles most likely to be found in a population, and where are common alleles found?

Rare alleles are mostly found in heterozygotes. Common alleles are mostly found in homozygotes.

What type of selection is occurring if the absolute fitness values are W_A1A1 = W_A1A2 > W_A2A2

• Selection against a recessive allele (A2)

What is the difference between absolute fitness (W) and relative fitness (w)?

• Absolute fitness is a direct measurable quantity (e.g., number of eggs). Relative fitness is a unitless comparison scaled relative to the most successful genotype.

How do you calculate relative fitness (w) for a genotype?

Divide its absolute fitness by the absolute fitness of the most successful genotype (w = W/W_max)

What is the relative fitness (w) value of the most successful genotype in a population?

• It is always exactly 1.

What is the fundamental cause of genetic drift?

Random sampling error during reproduction in finite populations.

Is genetic drift limited only to population bottlenecks and founder effects?

No. It is always happening in every population because no real-world population is infinite.

How do population size and long-term generations influence genetic drift?

Drift is much stronger in small populations and over many generations it tends to remove genetic variation (often eliminating the rarer allele).

What is the term for a mating pattern where individuals with similar phenotypes mate preferentially, but only for a specific trait?

Assortative mating ("like with like" at one trait).

What is the term for a mating pattern where individuals preferentially choose mates with different phenotypes for a specific trait?

Disassortative mating ("opposites attract" at one trait).

What type of non-random mating involves preferential mating among closely related individuals, affecting genotypes across the entire genome?

• Inbreeding ("like with like" genome-wide).

What is the genome-wide equivalent of "opposites attract" mating?

Inbreeding avoidance (preferring unrelated mates genome-wide).

Why do rare alleles increase in frequency under a heterozygote advantage scenario?

Because rare alleles are mostly found in heterozygotes, which have the highest fitness advantage over homozygotes.

Why do rare alleles rapidly decrease in frequency under a heterozygote disadvantage scenario?

Because rare alleles reside almost exclusively in heterozygotes, meaning they are constantly selected against and can be quickly wiped out.

If a population with 250 A1A1, 500 A1A2, and 250 A2A2 begins mating perfectly assortatively, what offspring proportions will the 500 heterozygotes produce?

• They will produce 250 heterozygotes A1A2 and 125 of each homozygote A1A1 and A2A2

Does assortative mating cause a population to evolve? Why or why not?

No. Assortative mating changes genotype frequencies (reducing heterozygotes) and perturbs Hardy-Weinberg equilibrium, but it does not alter allele frequencies on its own.

Does inbreeding (mating with close relatives) directly cause evolution?

No. Like assortative mating, inbreeding rearranges alleles into genotypes but does not change the overall allele frequencies in the gene pool.

If inbreeding does not cause evolution, why are harmful genetic disorders so much more common in inbred populations and royal dynasties?

It drastically increases homozygosity, giving offspring a much higher chance of inheriting two copies of rare, harmful recessive alleles from their related parents.