Further Thought

Home > Population Genetics 3: Multiple Loci and Sex > Further Thought

Further Thought

Use the questions at the end of the chapter to explore concepts and connections in greater depth through application and synthesis.

1. In horses, the basic color of the coat is governed by the E locus. EE and Ee horses can make black pigment, while ee horses are a reddish chestnut. A different locus, the R locus, can cause roan, a scattering of white hairs throughout the basic coat color. However, the roan allele has a serious drawback: RR embryos always die during fetal development. Rr embryos survive and are roan, while rr horses survive and are not roan. The E locus and the R locus are tightly linked.

Suppose that several centuries ago, a Spanish galleon with a load of conquistadors' horses was shipwrecked by a large grassy island. Just by chance, the horses that survived the shipwreck and swam to shore were 20 chestnut roans (eeRr) and 20 nonroan homozygous blacks (EErr). On the island, they interbred with each other and established a wild population. The island environment exerts no direct selection on the E locus.

What was D, the coefficient of linkage disequilibrium, in the initial population of 20 horses? Was the initial population in linkage equilibrium or not? If not, what chromosomal genotypes were underrepresented?
Do you expect the frequency of the chestnut allele, e, to increase or decrease in the first crop of foals? Would your answer be different if the founding population had been just 10 horses (5 of each color)? Explain your reasoning.
If you could travel to this island today, can you predict what D would be now? Do you have predictions about whether more horses will be roan versus nonroan, or chestnut versus black? If not, explain what further information you would need. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. Imagine a population of pea plants that is in linkage equilibrium for two linked loci, flower color (P purple, p red) and pollen shape L long, l round).

What sort of selection event would create linkage disequilibrium? For example, will selection at just one locus (e.g., all red-flowered plants die) create linkage disequilibrium? How about selection at two loci (e.g., red-flowered plants die, and long-pollen plants die)? How about selection on a certain combination of genotypes at two loci (e.g., only plants that are both red-flowered and have long pollen grains die)?
Now imagine a population that is already in linkage disequilibrium for these two loci. Will selection for purple flowers affect evolution of pollen shape? How is your answer different from your answer to part a, and why? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. Figure 7.3 shows how selection on multilocus genotypes can create linkage disequilibrium. From the postselection population in Figure 7.3b, develop a bar graph like the ones in Figure 7.2. Does this bar graph confirm that the postselection population is in linkage disequilibrium? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. Box 7.1 shows that when and then Show that when (Hint: p, the frequency of allele A, is equal to Likewise, s, the frequency of allele B, is equal to Multiply these quantities and simplify the expression. Knowing that will allow you to make a key substitution.) [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

5. Populations of rats exposed to the poison warfarin rapidly evolve resistance. The gene for warfarin resistance is located on rat chromosome 1. Michael Kohn and colleagues (2000) surveyed rats in five German rat populations known to vary in their recent exposure to warfarin and in their resistance. The researchers determined the genotype of each rat at a number of maker loci near the warfarin resistance gene. For each population, the researchers calculated the average heterozygosity (H) among the marker loci, the fraction of loci that were out of Hardy–Weinberg equilibrium (HWE), and the fraction of marker-locus pairs that were in linkage disequilibrium (LD). Their results appear in Figure 7.24. Based on these graphs, rank the five populations in order, from lowest to highest, for exposure to warfarin and resistance. Explain your reasoning. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

6.

In the beetle evolution experiment (Figure 7.18), Dunbrack et al. did not actually use sexual beetles and asexual beetles. Instead, they used two colors of sexual beetles, but forced one color of beetles to grow in population size as if it were asexual. They also did the experiment again with the other color as "asexual." Why was it important that the researchers run the experiment both ways? Compare the graphs of the two different sets of experiments (red asexual, and black asexual). Did the two strains of beetles perform differently?
Actual asexual beetles would reproduce twice as fast as sexual beetles, but in the experiment the authors made the asexual beetles reproduce three times as fast. Why do you think they did this?
The researchers' simulated asexual population was not allowed to evolve at all in response to selection imposed by competition. Is this different from what would have happened in an actual population of asexual beetles? Do you think Dunbrack et al.'s experiment is a valid test of asexual reproduction versus sexual reproduction? Briefly describe the next experiment that you think Dunbrack et al. should do to follow up on this topic. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

7. In 1992, Spolsky, Phillips, and Uzzell reported genetic evidence that asexually reproducing lineages of a salamander species have persisted for about 5 million years, an unusually long time. Is this surprising? Why or why not? Speculate about what sort of environment these asexual salamanders live in, and whether their population sizes are typically small (say, under 100) or large (say, over 1,000). [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

8. Volvox (Figure 7.16a) are abundant and active in lakes during the spring and summer. During winter they are inactive, existing in a resting state in encysted zygotes called zygospores. During most of the spring and summer, Volvox reproduce asexually; but at times they switch and reproduce sexually instead. When would you predict that Volvox would be sexual: spring, early summer, or late summer? Explain your reasoning. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	In horses, the basic color of the coat is governed by the E locus. EE and Ee horses can make black pigment, while ee horses are a reddish chestnut. A different locus, the R locus, can cause roan, a scattering of white hairs throughout the basic coat color. However, the roan allele has a serious drawback: RR embryos always die during fetal development. Rr embryos survive and are roan, while rr horses survive and are not roan. The E locus and the R locus are tightly linked. Suppose that several centuries ago, a Spanish galleon with a load of conquistadors' horses was shipwrecked by a large grassy island. Just by chance, the horses that survived the shipwreck and swam to shore were 20 chestnut roans (eeRr) and 20 nonroan homozygous blacks (EErr). On the island, they interbred with each other and established a wild population. The island environment exerts no direct selection on the E locus. What was D, the coefficient of linkage disequilibrium, in the initial population of 20 horses? Was the initial population in linkage equilibrium or not? If not, what chromosomal genotypes were underrepresented? Do you expect the frequency of the chestnut allele, e, to increase or decrease in the first crop of foals? Would your answer be different if the founding population had been just 10 horses (5 of each color)? Explain your reasoning. If you could travel to this island today, can you predict what D would be now? Do you have predictions about whether more horses will be roan versus nonroan, or chestnut versus black? If not, explain what further information you would need. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	Imagine a population of pea plants that is in linkage equilibrium for two linked loci, flower color (P purple, p red) and pollen shape L long, l round). What sort of selection event would create linkage disequilibrium? For example, will selection at just one locus (e.g., all red-flowered plants die) create linkage disequilibrium? How about selection at two loci (e.g., red-flowered plants die, and long-pollen plants die)? How about selection on a certain combination of genotypes at two loci (e.g., only plants that are both red-flowered and have long pollen grains die)? Now imagine a population that is already in linkage disequilibrium for these two loci. Will selection for purple flowers affect evolution of pollen shape? How is your answer different from your answer to part a, and why? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Figure 7.3 shows how selection on multilocus genotypes can create linkage disequilibrium. From the postselection population in Figure 7.3b, develop a bar graph like the ones in Figure 7.2. Does this bar graph confirm that the postselection population is in linkage disequilibrium? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	Box 7.1 shows that when and then Show that when (Hint: p, the frequency of allele A, is equal to Likewise, s, the frequency of allele B, is equal to Multiply these quantities and simplify the expression. Knowing that will allow you to make a key substitution.) [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



5.	Populations of rats exposed to the poison warfarin rapidly evolve resistance. The gene for warfarin resistance is located on rat chromosome 1. Michael Kohn and colleagues (2000) surveyed rats in five German rat populations known to vary in their recent exposure to warfarin and in their resistance. The researchers determined the genotype of each rat at a number of maker loci near the warfarin resistance gene. For each population, the researchers calculated the average heterozygosity (H) among the marker loci, the fraction of loci that were out of Hardy–Weinberg equilibrium (HWE), and the fraction of marker-locus pairs that were in linkage disequilibrium (LD). Their results appear in Figure 7.24. Based on these graphs, rank the five populations in order, from lowest to highest, for exposure to warfarin and resistance. Explain your reasoning. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



6.	In the beetle evolution experiment (Figure 7.18), Dunbrack et al. did not actually use sexual beetles and asexual beetles. Instead, they used two colors of sexual beetles, but forced one color of beetles to grow in population size as if it were asexual. They also did the experiment again with the other color as "asexual." Why was it important that the researchers run the experiment both ways? Compare the graphs of the two different sets of experiments (red asexual, and black asexual). Did the two strains of beetles perform differently? Actual asexual beetles would reproduce twice as fast as sexual beetles, but in the experiment the authors made the asexual beetles reproduce three times as fast. Why do you think they did this? The researchers' simulated asexual population was not allowed to evolve at all in response to selection imposed by competition. Is this different from what would have happened in an actual population of asexual beetles? Do you think Dunbrack et al.'s experiment is a valid test of asexual reproduction versus sexual reproduction? Briefly describe the next experiment that you think Dunbrack et al. should do to follow up on this topic. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



7.	In 1992, Spolsky, Phillips, and Uzzell reported genetic evidence that asexually reproducing lineages of a salamander species have persisted for about 5 million years, an unusually long time. Is this surprising? Why or why not? Speculate about what sort of environment these asexual salamanders live in, and whether their population sizes are typically small (say, under 100) or large (say, over 1,000). [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



8.	Volvox (Figure 7.16a) are abundant and active in lakes during the spring and summer. During winter they are inactive, existing in a resting state in encysted zygotes called zygospores. During most of the spring and summer, Volvox reproduce asexually; but at times they switch and reproduce sexually instead. When would you predict that Volvox would be sexual: spring, early summer, or late summer? Explain your reasoning. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 7: Population Genetics 3: Multiple Loci and Sex

Further Thought