En anglais

Forces that hold variation in balance

Science · Genetics & Evolution · ages 18-20

Name ______________________ Date ____________

Which force alone creates brand new alleles?
- Drift
- Migration
- Mutation
Which set of conditions keeps allele frequencies constant in the Hardy-Weinberg model?
- Mutation, migration, nonrandom mating, a small population, and strong selection all act together
- No mutation, random mating, no gene flow, a very large population, and no selection
- Migrants arrive and selection favors one allele
Under Hardy-Weinberg assumptions, p squared plus 2pq plus q squared equals 1 describes the no-force genotype-frequency baseline. True or false?
Circle one: True False
Two similarly sized populations exchange migrants every generation. What usually happens over time?
- Both populations lose all variation after a few generations
- Their allele frequencies move toward each other
- Mutation stops completely in both populations
Why can a harmful recessive allele linger instead of vanishing when unaffected heterozygotes carry it?
- It always helps carriers survive better than every other genotype
- Most copies hide in unaffected heterozygotes
- Selection favors every rare allele regardless of its effect
Assuming Hardy-Weinberg conditions and a fully recessive trait, 4 in 100 people show the trait. Type q, the recessive allele frequency.
Answer: ______________
A harmful allele is removed by selection each generation, yet it stays at a similar frequency. Migrants arrive every generation from a population where the allele is common. What could maintain it?
- Gene flow adds copies
- The square-root calculation fails for every recessive trait
- Homozygotes no longer reproduce in any population
In this lesson's model, which statement best compares mutation with gene flow?
- Mutation always changes frequencies faster than migration, regardless of population size and input rate
- Gene flow can never change allele frequencies, even when migrants arrive each generation
- Mutation usually adds new alleles slowly, while enough migrant input can change frequencies faster

Answer key

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

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Mutation · Mutation is the only source of new alleles; the other forces move existing variation around.
No mutation, random mating, no gene flow, a very large population, and no selection · The model requires all five listed conditions. Breaking any one can change allele or genotype frequencies.
True · Under the model conditions, p squared, 2pq, and q squared are the expected genotype frequencies and sum to 1.
Their allele frequencies move toward each other · Gene flow mixes existing alleles, so repeated migration usually narrows the frequency difference between similarly sized populations.
Most copies hide in unaffected heterozygotes · Selection removes affected homozygotes, but most copies can remain in unaffected heterozygotes.
0.2 · Affected people are aa, so q squared is 4 divided by 100, or 0.04. The square root of 0.04 is 0.2.
Gene flow adds copies · Gene flow can add the allele as selection removes it, and a similar frequency can result when input and removal are balanced. Recurrent mutation can also supply copies, as the lesson notes, but it is not among the choices.
Mutation usually adds new alleles slowly, while enough migrant input can change frequencies faster · Mutation is usually a slow source of new alleles. Gene flow can change frequencies faster when migrant input is large relative to the population.