Answers to Concept Checks
- 1. There are fewer alleles than genotypes, so the gene pool can be described by fewer parameters when allelic frequencies are used. Additionally, the genotypes are temporary assemblages of alleles that break down each generation; the alleles are passed from generation to generation in sexually reproducing organisms.
- 2. a
- 3. c
- 4. 0.10
- 5. b
- 6. c
- 7. c
- 8. d
- 9. b
- 10. In overdominance, the heterozygote has the highest fitness and both alleles are maintained. In directional selection, selection causes one allele or trait to increase in frequency.
WORKED PROBLEMS
Problem
A recessive allele for red hair (r) has a frequency of 0.2 in population I and a frequency of 0.01 in population II. A famine in population I causes a number of people in population I to migrate to population II, where they reproduce randomly with the members of population II. Geneticists estimate that, after migration, 15% of the people in population II consist of people who migrated from population I. What will be the frequency of red hair in population II after the migration?
Solution Strategy
What information is required in your answer to the problem?
The frequency of red hair in population II after the migration.
What information is provided to solve the problem?
The allele for red hair (r) is recessive.-
The initial frequency of r in population I is 0.2.
-
The initial frequency of r in population II is 0.01.
-
After migration, 15% of the people in population II consist of people who migrated from population I.
For help with this problem, review:
Migration in Section 25.4.
Solution Steps
From Equation 25.14, the allelic frequency in a population after migration (q′II) is
q′II = qI(m) + q(1 − m)
where qI and qII are the allelic frequencies in population I (migrants) and population II (residents), respectively, and m is the proportion of population II that consist of migrants. In this problem, the frequency of red hair is 0.2 in population I and 0.01 in population II. Because 15% of population II consists of migrants, m = 0.15. Substituting these values into Equation 25.14, we obtain
q′II = 0.2(0.15) + (0.01)(1 − 0.15) = 0.03 + 0.0085 = 0.0385
which is the expected frequency of the allele for red hair in population II after migration. Red hair is a recessive trait; if mating is random for hair color, the frequency of red hair in population II after migration will be
f(rr) = q2 = (0.0385)2 = 0.0015
Recall: With random mating the expected frequencies of the genotypes are p2 (RR), 2pq(Rr) and q2(rr).