Chapter summary
Population genetics, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
Count alleles, not just organisms
How can 200 organisms contain 400 copies of one gene?
Key idea and reminders
At a diploid autosomal locus, each individual contributes two alleles: p = (2AA + Aa) / (2N).
- Use 2N allele copies for a diploid autosomal locus.
- Directly counting known genotypes does not need Hardy-Weinberg equilibrium.
- Dominant describes expression in a heterozygote, not how common an allele is.
Keep in mind: 0.15 is the observed aa genotype frequency. The a allele also occurs in every Aa individual. Count all a copies, or use a stated equilibrium model when genotypes are unknown.
02
Build the Hardy-Weinberg model
Why is the heterozygote frequency 2pq?
Key idea and reminders
Random combination of gametes gives AA = p squared, Aa = 2pq and aa = q squared, when the model applies.
- p + q = 1 counts alleles; p squared + 2pq + q squared = 1 counts genotypes.
- The factor 2 comes from the two ways to make Aa.
- Phenotype frequencies depend on the dominance relationship.
Keep in mind: Aa and aA are the same genotype in this autosomal model. The two products count the two possible parental origins of the alleles.
03
Work backwards from a recessive phenotype
If 9% show a recessive phenotype, are 9% of the alleles recessive?
Key idea and reminders
Under Hardy-Weinberg equilibrium and complete dominance, the recessive phenotype frequency is q squared; take its square root to find q.
- Change percent to proportion before taking a square root.
- Carriers are Aa: use 2pq under equilibrium.
- A conditional proportion uses the named subgroup as its denominator.
Keep in mind: p is the A allele frequency. Under complete dominance the A phenotype includes two genotypes, AA and Aa, giving p squared + 2pq under equilibrium.
04
Know when the model applies
Does a different genotype ratio prove natural selection?
Key idea and reminders
The model assumes random mating, a very large population, no selection, no mutation and no migration at the locus being studied.
- State the five assumptions and explain their biological meaning.
- Non-random mating can alter genotype proportions without changing allele frequencies.
- A departure does not by itself identify which assumption failed.
Keep in mind: Sampling variation and several population processes can create a mismatch. Evidence about survival, reproduction and competing explanations is needed before attributing it to selection.
05
Why harmful alleles can persist
Why does selection not remove every harmful recessive allele?
Key idea and reminders
Selection acts through phenotypes. A recessive allele can persist in heterozygotes, and some environments favour heterozygotes over either homozygote.
- A recessive phenotype can be masked in heterozygotes.
- Heterozygote advantage can maintain both alleles when it raises reproductive success.
- The effect of an allele depends on genotype and environment.
Keep in mind: Populations do not plan. Alleles persist through inheritance, mutation, migration and the present effects of chance and selection.