Selection as bookkeeping

Science · Genetics & Evolution · ages 18-19

Name ______________________ Date ____________

Genotypes AA, Aa, and aa start at frequencies 0.25, 0.50, and 0.25. Their fitnesses are 1.0, 0.5, and 0. What is the frequency of A after selection?
Answer: ______________
What does relative fitness measure?
- The number of years an individual lives
- Expected reproductive contribution compared with the best genotype
- The total number of offspring produced by the whole population in one generation
Selection can easily remove a rare recessive allele in a few generations when heterozygotes are unaffected.
Circle one: True False
How is mean fitness calculated?
- Allele count divided by the number of individuals
- Subtract the worst genotype's fitness from the best genotype's fitness
- Multiply each frequency by fitness, then add the products
Under Hardy-Weinberg conditions, a fully recessive allele has frequency q = 0.1. What fraction of the population is expected to be affected?
Answer: ______________
Why does selection against a rare recessive allele slow down when unaffected heterozygotes carry it?
- Fitness values always shrink toward zero whenever any allele becomes rare in a population
- Homozygotes become scarce, so copies hide in unaffected carriers
- New mutations rebuild the harmful allele in every generation
In a haploid population, strain A starts at frequency 0.4 with fitness 1.2, while strain a starts at frequency 0.6 with fitness 0.8. Type the next frequency of A.
Answer: ______________
Alleles A and a are each at frequency 0.5, and genotypes start in Hardy-Weinberg proportions. Fitnesses are 0.8 for AA, 1.0 for Aa, and 0.8 for aa. What is the frequency of A after one generation of selection?
Answer: ______________
Genotype X leaves 6 offspring where the best leaves 8. What is its fitness?
- 0.75
- 1.33
- 6
A rare harmful allele is lethal in homozygotes, and carriers also pay a small fitness cost. Compared with a fully recessive allele, what happens to its removal as it becomes rare?
- Removal stalls even sooner, because carriers still hide every copy from selection
- Removal stops once the allele falls below one percent
- Removal keeps going, because selection also acts on carriers

Answer key

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

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0.75 · The weighted genotype contributions are 0.25 times 1.0 plus 0.50 times 0.5 plus 0.25 times 0, giving mean fitness 0.50. The normalized AA frequency is 0.50, and half of the normalized Aa frequency is 0.25, so the next A frequency is 0.75.
Expected reproductive contribution compared with the best genotype · Relative fitness compares a genotype's expected reproductive contribution with the best genotype in the same population.
False · If heterozygotes are unaffected, most copies hide in them as the allele becomes rare. Selection sees fewer affected homozygotes, so removal slows.
Multiply each frequency by fitness, then add the products · Mean fitness is the fitness-weighted sum of genotype frequencies.
0.01 · Under Hardy-Weinberg conditions, affected recessive homozygotes have frequency q squared. With q equal to 0.1, q squared is 0.01.
Homozygotes become scarce, so copies hide in unaffected carriers · When the allele is rare, homozygotes occur at about q squared. If carriers are unaffected, most copies are hidden from selection in heterozygotes.
0.5 · Mean fitness is 0.4 times 1.2 plus 0.6 times 0.8, which is 0.48 plus 0.48, or 0.96. Divide A's weighted share, 0.48, by 0.96 to get 0.5.
0.5 · Hardy-Weinberg proportions give 0.25 AA, 0.5 Aa, and 0.25 aa. The weighted frequencies are 0.2, 0.5, and 0.2, so mean fitness is 0.9. The new A frequency is 0.2 divided by 0.9 plus half of 0.5 divided by 0.9, which is 0.45 divided by 0.9, or 0.5.
0.75 · Divide by the best genotype's count: 6 divided by 8 is 0.75. Dividing 8 by 6 gives 1.33, which flips the comparison, and 6 is an absolute count, not a relative score.
Removal keeps going, because selection also acts on carriers · A carrier cost means selection acts on heterozygotes, which hold most copies of a rare allele. Removal therefore does not stall as the allele becomes rare, unlike a fully recessive allele whose copies sit in unaffected carriers.