Further Thought

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Further Thought

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

1. Black color in horses is governed primarily by a recessive allele at the A locus. AA and Aa horses are nonblack colors such as bay, while aa horses are black all over. (Other loci can override the effect of the A locus, but we will ignore that complication.) A few years ago, a reader of the Usenet newsgroup "rec.equestrian" asked why there are relatively few black horses of the Arabian breed. One response was "Black is a rare color because it is recessive. More Arabians are bay or gray because those colors are dominant." What is wrong with this explanation? (Assume that the A and a alleles are in Hardy–Weinberg equilibrium, which was probably true at the time of this discussion.) Generally, what does the Hardy–Weinberg model show us about the impact that an allele's dominance or recessiveness has on its frequency? Should an allele become more common (or less common) simply because it is dominant (or recessive)? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. In humans, the COL1A1 locus codes for a certain collagen protein found in bone. The normal allele at this locus is denoted with S. A recessive allele s is associated with reduced bone mineral density and increased risk of fractures in both Ss and ss women. A recent study of 1,778 women showed that 1,194 were SS, 526 were Ss, and 58 were ss (Uitterlinden et al. 1998).

Are these two alleles in Hardy–Weinberg equilibrium in this population? How do you know? What information would you need to determine whether the alleles will be in Hardy–Weinberg equilibrium in the next generation? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. We used Figure 5.13 as an example of how the frequency of an allele (in fruit flies) does not change in unselected (control) populations, but does change in response to selection. However, look again at the unselected control lines in Figure 5.13. The frequency of the allele in the two control populations did change a little, moving up and down over time. Which assumption of the Hardy–Weinberg model is most probably being violated? If this experiment were repeated, what change in experimental design would reduce this deviation from Hardy–Weinberg equilibrium? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. Most animal populations have a 50:50 ratio of males to females. This does not have to be so; it is theoretically possible for parents to produce predominantly male offspring or predominantly female offspring. Imagine a monogamous population with a male-biased sex ratio, say, 70% males and 30% females. Which sex will have an easier time finding a mate? As a result, which sex will probably have higher average fitness? Which parents will have higher fitness—those that produce mostly males or those that produce mostly females? Now imagine the same population with a female-biased sex ratio, and answer the same questions. What sort of selection is probably maintaining the 50:50 sex ratio seen in most populations? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

5. Discuss how each of the following recent developments may affect the frequency of alleles that cause cystic fibrosis (CF).

Many women with CF now survive long enough to have children. (CF causes problems with reproductive ducts, but many CF women can bear children nonetheless. CF men are usually sterile.)
Typhoid fever in developed nations has declined to very low levels since 1900.
In some populations, couples planning to have children are now routinely screened for the most common CF alleles.
Drug-resistant typhoid fever has recently appeared in several developing nations. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

6. Consider what makes a new mutant allele dominant or recessive. To guide your thinking, imagine an enzyme that changes substance A to substance B. If B is a nutrient that is needed only in minimal amounts, will a loss-of-function mutation be dominant or recessive? If A is a toxin that must be entirely broken down, will a loss-of-function mutation be dominant or recessive? How about a new mutant allele that results in a form of the protein that can catalyze an entirely new reaction (say, from A to new substance C?) Can you think of other examples of protein function that will affect whether a new allele is dominant or recessive? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

7. There are two common alleles for the human muscle enzyme ACE (angiotensin-converting enzyme)—a shorter D allele, and a longer I allele that has an insertion of 287 base pairs. The ACE coded by the I allele has lower activity, but it is also associated with superior muscular performance after physical training. One study (Williams et al. 2000) of 35 II and 23 DD men found that though they didn't differ in muscular efficiency before training, after 11 weeks of aerobic training, the II homozygotes had 8% greater muscular efficiency. The I allele is also associated with greater endurance and greater muscular growth after strength training. Speculate on why the D allele still remains at relatively high frequency in the human population. How could you test your ideas? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

8. In our discussion of Jaeken syndrome in Section 5.2, we asserted that parents who are both carriers of the R141H allele can expect a different distribution of phenotypes among their children than parents who are carriers of two different disease alleles. Explain the logic behind this assertion. What genotypes and phenotypes, and in what ratios, should the following pairs of parents expect among their liveborn children? What would you tell these parents if you were a genetic counselor?

Mother's genotype: father's genotype:
Mother's genotype: father's genotype:
Mother's genotype: father's genotype: [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

9. Kerstin Johannesson and colleagues (1995) studied two populations of a marine snail living in the intertidal zone on the shore of Ursholmen Island. Each year, the researchers determined the allele frequencies for the enzyme aspartate aminotransferase (don't worry about what this enzyme does). Their data are shown in the graphs in Figure 5.28. The first year of the study was 1987. In 1988, a bloom of toxic algae (pink bands) killed all of the snails in the intertidal zone across the entire island. That is why there are no data for 1988 and 1989. Although the snails living in the intertidal zone were exterminated by the bloom, snails of the same species living in the splash zone just above the intertidal survived unscathed. By 1990, the intertidal zone had been recolonized by splash-zone snails. Your challenge in this question is to develop a coherent explanation for the data in the graphs. In each part, be sure to name the evolutionary mechanism involved (selection, mutation, migration, or drift).

Why was the frequency of the allele higher in both populations in 1990 than it was in 1987? Name the evolutionary mechanism, and explain.
Why did the allele frequency decline in both populations from 1990 through 1993? Name the evolutionary mechanism, and explain.
Why are the curves traced by the 1990–1993 data for the two populations generally similar, but not exactly identical? Name the evolutionary mechanism, and explain.
Predict what would happen to the allele frequencies if we followed these two populations for another 100 years (assuming there are no more toxic algal blooms). Explain your reasoning. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	Black color in horses is governed primarily by a recessive allele at the A locus. AA and Aa horses are nonblack colors such as bay, while aa horses are black all over. (Other loci can override the effect of the A locus, but we will ignore that complication.) A few years ago, a reader of the Usenet newsgroup "rec.equestrian" asked why there are relatively few black horses of the Arabian breed. One response was "Black is a rare color because it is recessive. More Arabians are bay or gray because those colors are dominant." What is wrong with this explanation? (Assume that the A and a alleles are in Hardy–Weinberg equilibrium, which was probably true at the time of this discussion.) Generally, what does the Hardy–Weinberg model show us about the impact that an allele's dominance or recessiveness has on its frequency? Should an allele become more common (or less common) simply because it is dominant (or recessive)? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	In humans, the COL1A1 locus codes for a certain collagen protein found in bone. The normal allele at this locus is denoted with S. A recessive allele s is associated with reduced bone mineral density and increased risk of fractures in both Ss and ss women. A recent study of 1,778 women showed that 1,194 were SS, 526 were Ss, and 58 were ss (Uitterlinden et al. 1998). Are these two alleles in Hardy–Weinberg equilibrium in this population? How do you know? What information would you need to determine whether the alleles will be in Hardy–Weinberg equilibrium in the next generation? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	We used Figure 5.13 as an example of how the frequency of an allele (in fruit flies) does not change in unselected (control) populations, but does change in response to selection. However, look again at the unselected control lines in Figure 5.13. The frequency of the allele in the two control populations did change a little, moving up and down over time. Which assumption of the Hardy–Weinberg model is most probably being violated? If this experiment were repeated, what change in experimental design would reduce this deviation from Hardy–Weinberg equilibrium? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	Most animal populations have a 50:50 ratio of males to females. This does not have to be so; it is theoretically possible for parents to produce predominantly male offspring or predominantly female offspring. Imagine a monogamous population with a male-biased sex ratio, say, 70% males and 30% females. Which sex will have an easier time finding a mate? As a result, which sex will probably have higher average fitness? Which parents will have higher fitness—those that produce mostly males or those that produce mostly females? Now imagine the same population with a female-biased sex ratio, and answer the same questions. What sort of selection is probably maintaining the 50:50 sex ratio seen in most populations? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



5.	Discuss how each of the following recent developments may affect the frequency of alleles that cause cystic fibrosis (CF). Many women with CF now survive long enough to have children. (CF causes problems with reproductive ducts, but many CF women can bear children nonetheless. CF men are usually sterile.) Typhoid fever in developed nations has declined to very low levels since 1900. In some populations, couples planning to have children are now routinely screened for the most common CF alleles. Drug-resistant typhoid fever has recently appeared in several developing nations. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



6.	Consider what makes a new mutant allele dominant or recessive. To guide your thinking, imagine an enzyme that changes substance A to substance B. If B is a nutrient that is needed only in minimal amounts, will a loss-of-function mutation be dominant or recessive? If A is a toxin that must be entirely broken down, will a loss-of-function mutation be dominant or recessive? How about a new mutant allele that results in a form of the protein that can catalyze an entirely new reaction (say, from A to new substance C?) Can you think of other examples of protein function that will affect whether a new allele is dominant or recessive? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



7.	There are two common alleles for the human muscle enzyme ACE (angiotensin-converting enzyme)—a shorter D allele, and a longer I allele that has an insertion of 287 base pairs. The ACE coded by the I allele has lower activity, but it is also associated with superior muscular performance after physical training. One study (Williams et al. 2000) of 35 II and 23 DD men found that though they didn't differ in muscular efficiency before training, after 11 weeks of aerobic training, the II homozygotes had 8% greater muscular efficiency. The I allele is also associated with greater endurance and greater muscular growth after strength training. Speculate on why the D allele still remains at relatively high frequency in the human population. How could you test your ideas? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



8.	In our discussion of Jaeken syndrome in Section 5.2, we asserted that parents who are both carriers of the R141H allele can expect a different distribution of phenotypes among their children than parents who are carriers of two different disease alleles. Explain the logic behind this assertion. What genotypes and phenotypes, and in what ratios, should the following pairs of parents expect among their liveborn children? What would you tell these parents if you were a genetic counselor? Mother's genotype: father's genotype: Mother's genotype: father's genotype: Mother's genotype: father's genotype: [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



9.	Kerstin Johannesson and colleagues (1995) studied two populations of a marine snail living in the intertidal zone on the shore of Ursholmen Island. Each year, the researchers determined the allele frequencies for the enzyme aspartate aminotransferase (don't worry about what this enzyme does). Their data are shown in the graphs in Figure 5.28. The first year of the study was 1987. In 1988, a bloom of toxic algae (pink bands) killed all of the snails in the intertidal zone across the entire island. That is why there are no data for 1988 and 1989. Although the snails living in the intertidal zone were exterminated by the bloom, snails of the same species living in the splash zone just above the intertidal survived unscathed. By 1990, the intertidal zone had been recolonized by splash-zone snails. Your challenge in this question is to develop a coherent explanation for the data in the graphs. In each part, be sure to name the evolutionary mechanism involved (selection, mutation, migration, or drift). Why was the frequency of the allele higher in both populations in 1990 than it was in 1987? Name the evolutionary mechanism, and explain. Why did the allele frequency decline in both populations from 1990 through 1993? Name the evolutionary mechanism, and explain. Why are the curves traced by the 1990–1993 data for the two populations generally similar, but not exactly identical? Name the evolutionary mechanism, and explain. Predict what would happen to the allele frequencies if we followed these two populations for another 100 years (assuming there are no more toxic algal blooms). 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 5: Mendelian Genetics in Populations I: Selection and Mutation

Further Thought