Problem Set 8
Due in laboratory Nov. 9/10.
1. Lactose intolerance is recessive. Individuals homozygous for the base C at the DNA
position 13,910 base pairs upstream from the start of transcription of the lactase gene are
lactose intolerant (upstream refers to the location with respect to the direction that RNA
polymerase travels during transcription). Those individuals have genotype CC. Lactosetolerant individuals include heterozygotes with the CT genotype (one allele has the C base
and the other carries a T base at that position) and homozygotes with the TT genotype.
In Sweden, the allele frequency of T is 0.71.
a. The variant site is not located in a transcribed region of the lactase gene. How can it cause
b. Under the assumption of Hardy-Weinberg equilibrium, calculate the expected frequency
of each of the three genotype classes at this locus among Swedes (use a calculator).
c. Estimate the proportion of Swedish adults who can eat their herring with cream sauce,
worry-free. (In other words: what’s the expected frequency of the lactose-tolerant
2. An mRNA is represented below.
a. Indicate a one-base change that would produce a nonsense mutation.
b. Indicate a one-base change that would produce a silent mutation.
c. Indicate a one-base change that would produce a missense mutation.
3. Suppose the mutant allele responsible for X-linked color-blindness has a frequency in the
population of 0.1.
a. At Hardy-Weinberg equilibrium, what is the expected frequency of color-blindness among
b. What’s the expected frequency among men? (Think of the gene-pool metaphor.)
BONUS QUESTIONS! OPTIONAL EXTRA POINTS!
4a. In terms of genotypic and environmental variances, what is the definition of heritability?
(Think way back to lecture 10)
b. (This one looks hard. You’ll have to consider question 6a along with Hardy-Weinberg
Equilibrium.) Consider a quantitative trait affected by random environmental effects, like
Johannsen’s beans. The trait is also affected by the genotype at a single locus with two
alleles, A and a. The genotype at the locus has an additive effect on the phenotype: the
average phenotype of aa homozygous individuals is 1, while heterozygotes have a phenotype
of 2 on average, and AA homozygotes 3. (These are just averages, because the trait is also
affected by random environmental effects.)
How is the heritability of the trait is affected by the frequency of the A allele? Suppose that
the amount of environmental variance is constant, independent of the genotypes. It might
help to start by thinking about what the heritability would be if the frequency of A is 0 or 1.
5. New York City is home to pigeons and red-tailed hawks. In which species would you
expect genetic drift to cause greater variation in allele frequencies from generation to
generation? Explain. (One sentence should suffice.)