AP Biology: Unit 7, Population Genetics (Drill 30)

Question 1. Based on the phenotype data in Table 1, what is the frequency of the recessive allele (r) in this population?

• A) 0.36
• B) 0.60 ✓
• C) 0.40
• D) 0.18

Explanation: Correct answer: B. Under Hardy-Weinberg equilibrium, the frequency of the homozygous recessive genotype (rr) equals q². The frequency of black beetles = 180/500 = 0.36. Therefore q² = 0.36, and q = √0.36 = 0.60. (A) 0.36 is q², not q, the common error of forgetting to take the square root. (C) 0.40 is p (the dominant allele frequency), not q. (D) 0.18 is half the phenotype frequency and has no direct meaning in this context.

Question 2. Based on your calculation of allele frequencies, what is the expected frequency of heterozygous (Rr) beetles under Hardy-Weinberg equilibrium?

• A) 0.36
• B) 0.24
• C) 0.16
• D) 0.48 ✓

Explanation: Correct answer: D. The Hardy-Weinberg equation gives the heterozygote frequency as 2pq. With p = 0.40 and q = 0.60: 2(0.40)(0.60) = 0.48. Out of 500 beetles, 0.48 × 500 = 240 would be expected to be heterozygous. (A) 0.36 is q², the frequency of rr. (B) 0.24 is pq without the factor of 2. (C) 0.16 is p², the expected frequency of RR.

Question 3. After one generation of selection against black beetles (30% lower survival), which change in the population is most consistent with the principles of natural selection?

• A) The frequency of the r allele will decrease, and the frequency of the R allele will increase. ✓
• B) The frequency of the r allele will increase because recessive alleles hidden in heterozygotes are protected from selection.
• C) The frequencies of R and r will remain unchanged because selection only affects phenotypes, not allele frequencies.
• D) The frequency of homozygous dominant (RR) beetles will decrease to compensate for the loss of black beetles.

Explanation: Correct answer: A. Selection acts against the black phenotype (rr). Black beetles contribute fewer offspring to the next generation. Since all r alleles in rr beetles are exposed to selection, and rr beetles contribute fewer copies of r to the gene pool, the frequency of r decreases and R increases. (B) is partially true, heterozygotes do carry r alleles shielded from selection, but this slows r's decline; it does not cause r to increase. (C) is incorrect; selection on phenotype translates directly to differential allele transmission. (D) is incorrect; there is no compensatory mechanism reducing RR frequency; R alleles become more common, not less.

Question 4. A student argues: "Because the r allele is protected in heterozygotes, selection will never completely eliminate it from the population." Evaluate this claim.

• A) The claim is incorrect because heterozygotes do not carry the r allele.
• B) The claim is incorrect because selection against rr is strong enough to eliminate r in a single generation, thereby leaving no carriers and making future allele-frequency calculations unnecessary in the population over time.
• C) The claim is largely supported: as r becomes rarer, most remaining r alleles exist in heterozygotes where they are hidden from selection, making complete elimination by selection alone increasingly slow and difficult. ✓
• D) The claim is incorrect because mutation will eliminate r alleles independent of selection.

Explanation: Correct answer: C. As the frequency of a recessive allele declines under selection, the proportion found in heterozygotes (invisible to selection) increases relative to homozygotes. When r is rare, nearly all r alleles exist as Rr heterozygotes. Since selection only removes rr individuals, the rate of removal slows dramatically. Complete elimination by selection alone becomes increasingly slow and difficult, though in finite populations an allele can sometimes be lost to drift even when selection cannot drive it to zero. (A) is factually incorrect; heterozygotes (Rr) carry exactly one r allele. (B) is incorrect; 30% reduced survival is not sufficient to eliminate any allele in one generation. (D) is incorrect; mutation at typical rates does not efficiently eliminate existing alleles.

Question 5. If heterozygous beetles (Rr) had slightly higher reproductive success than either homozygote, this would be an example of which evolutionary mechanism, and what would be the long-term outcome?

• A) Directional selection; the R allele would eventually be fixed at a frequency of 1.0.
• B) Balancing selection (heterozygote advantage); both R and r alleles would be maintained at stable intermediate frequencies. ✓
• C) Disruptive selection; the population would split into two distinct subpopulations.
• D) Genetic drift; allele frequencies would fluctuate randomly around 0.50.

Explanation: Correct answer: B. When heterozygotes have greater fitness than either homozygote (overdominance), this is a form of balancing selection. The result is a stable equilibrium at which both alleles are maintained at intermediate frequencies. The classic example is sickle-cell anemia in malaria-endemic regions. (A) is incorrect; directional selection favors one extreme and would eventually fix one allele, the opposite of heterozygote advantage. (C) is incorrect; disruptive selection favors both extremes and disfavors the intermediate, the reverse of this scenario. (D) is incorrect; genetic drift is random and unrelated to fitness differences.