Selection Coefficients and Predicting Allele Frequency Change · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Selection as bookkeeping

Science · Genetics & Evolution · ages 18-19
Name ______________________   Date ____________

  1. In a haploid population, strain A starts at frequency 0.5 with fitness 1.5, while strain a has fitness 0.5. What is the frequency of A after one generation?

    Answer: ______________

  2. What does relative fitness measure?

    • Years an individual lives
    • Success against the best genotype
    • Babies counted in absolute numbers

  3. Selection can easily purge a rare recessive allele in a few generations.

    Circle one:   True   False

  4. How is mean fitness calculated?

    • Allele count divided by heads
    • Best fitness minus worst fitness
    • Each frequency times its fitness, added up

  5. Why does selection against a rare recessive allele slow down as the allele gets rarer?

    • Recessive alleles mutate back to dominant
    • Homozygous recessives grow rarer, so the allele hides in heterozygotes
    • Heterozygotes die out first
    • Fitness values change as alleles get rare

  6. Why does selection against a rare recessive slow down as it gets rarer?

    • Fitness values shrink toward zero
    • Homozygotes grow scarcer, so the allele hides
    • Mutations rebuild it each year

  7. Strain A at 0.5 has fitness 1.5 and strain a at 0.5 has fitness 0.5. Type the next frequency of A.

    Answer: ______________

  8. Alleles A and a are each at frequency 0.5. Fitnesses are 0.8 for AA, 1.0 for Aa, and 0.8 for aa. What is the frequency of A after one generation?

    Answer: ______________

  9. Genotype X leaves 6 offspring where the best leaves 8. What is its fitness?

    • 0.75
    • 6
    • 8

  10. A lethal recessive persists for centuries. Why?

    • It mutates back each generation
    • Heterozygotes die out first
    • Carriers hide it from selection
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Selection as bookkeeping W1-mt__JS2XJxOXV-s1

  1. 0.75 · Mean fitness is 0.5 x 1.5 + 0.5 x 0.5 = 1.0, so the new frequency is 0.75 / 1.0 = 0.75.
  2. Success against the best genotype · Each genotype is scored against the top performer.
  3. False · Hidden copies in heterozygotes make removal stall near zero.
  4. Each frequency times its fitness, added up · Mean fitness is the fitness weighted sum of genotype frequencies.
  5. Homozygous recessives grow rarer, so the allele hides in heterozygotes · Selection only sees the allele in homozygotes, and those grow scarce as the square of the frequency, so removal stalls.
  6. Homozygotes grow scarcer, so the allele hides · Homozygote frequency falls as the square, hiding copies in heterozygotes.
  7. 0.75 · Mean fitness is 1.0 and the A share is 0.75, so the answer is 0.75.
  8. 0.5 · Mean fitness is 0.25 x 0.8 + 0.5 x 1.0 + 0.25 x 0.8 = 0.9, and the A share is 0.2 + 0.25 = 0.45, so p = 0.45 / 0.9 = 0.5.
  9. 0.75 · Six divided by eight is 0.75, a purely relative score.
  10. Carriers hide it from selection · Nearly every copy rides safely inside healthy heterozygotes.
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