Anthropology 005
Problem Set # 2
Winter 2023
1. Often we can refine of our understanding of the way the world is by contrasting that with ways it is not
(sometimes called “counterfactuals”). Consider: i) a jar into which we pour equal amounts of yellow and blue
liquid, and ii) another jar into which we pour equal numbers of yellow and blue marbles. Put the lids on and
shake both jars. In the first jar we now see a green liquid. In the second we see a mosaic of yellow and blue
marbles. When Gregor Mendel was alive and studying pea plants, people thought that heredity was like the
first jar (blending), but Mendel proved that is was more like the second (particulate).
A. There is evidence from both Mendel’s F1 and, separately, from his F2 generation that supports his
particulate interpretation. What would each generation have been like if inheritance were blending, and what
did Mendel actually observe?
B. In additional experiments not discussed in your textbook, Mendel took some of his F1 hybrids and,
instead of breeding them among each other, he did what is called a backcross, breeding them with their
homozygote recessive parents. What does the particulate model predict about the offspring of that backcross?
(Hint: construct the appropriate Punnett square.)
C. In particulate hereditary systems like the one Mendel described, “variation is conserved.” What does that
phrase mean? (Hint: what happened in the F2 generation.)
D. If (counterfactually) inheritance were blending, would variation be conserved; and would that have any
effect on natural selection? Explain your answer.
2. Figure 3-3 of your textbook illustrates how meiosis produces sex cells. There are two important features of
meiosis: a reduction from a diploid to a haploid state, and recombination—the exchange of genetic material
between paternally derived and maternally derived chromosomes.
A. What is the effect of recombination on the diversity of an individual’s haploid gametes?
B. Recombination affects linkage and the composition of haplotypes. Alleles are said to be linked if they pass
from one generation to the next together, and these “fellow travelers” constitute a haplotype. To illustrate
these points let’s look at panel 1 of Figure 3-3. There, alleles A, B, and C are linked because they arrived
together in this individual from her father; and likewise, alleles A’, B’, and C’ are linked because they arrived
together from her mother. Thus, she was born with two different Chromosome 1 haplotypes, the one
containing A, B, and C, and the one containing A’, B’, and C’. The rest of Figure 3-3 shows the formation of
her gametes (sex cells). When she produced those gametes, why were B and C more likely to remain linked
to each other than to A, that is, why were B and C more likely to remain on the same haplotype? (Hint:
Assume the recombination sites are randomly distributed along the chromosome.)
C. In every generation recombination forms new haplotypes. How do those new haplotypes compare to the
haplotypes of the previous generation? (They do differ in one systematic way; what is that difference?)
3. The “stuff of heredity” would have to do two things: i) make very accurate copies of itself and ii) include
instructions for building phenotypes. DNA does both of those, and base-pairing rules are central to both
processes.
A. Explain the role of base pairing rules in DNA copying.
B. Explain the roles of base pairing rules in DNA’s assembly of phenotypes.
4. In Problem set 1 you were asked to distinguish between an evolutionary change and a developmental
change. That distinction is important to understand. However, it is also critical to realize that adaptive
developmental changes have been “preprogrammed” by the prior operation of natural selection (in previous
generations).
A. Facultative adaptations, like suntanning, are examples of that kind adaptive developmental change. What
does the word “adaptive” mean in that sentence?
B. What is suntanning adapted to; what were the relevant selection pressures that shaped this facultative
adaptation over evolutionary time?
C. What is the environmental trigger for suntanning, and does it make sense for this facultative adaptation to
be tied to that feature of the environment? Explain.
