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Ch. 26 - Population and Evolutionary Genetics
All textbooksKlug 12th EditionCh. 26 - Population and Evolutionary GeneticsProblem 19
Chapter 25, Problem 19

A botanist studying water lilies in an isolated pond observed three leaf shapes in the population: round, arrowhead, and scalloped. Marker analysis of DNA from 125 individuals showed the round-leaf plants to be homozygous for allele r1, while the plants with arrowhead leaves were homozygous for a different allele at the same locus, r2. Plants with scalloped leaves showed DNA profiles with both the r1 and r2 alleles. Frequency of the r1 allele was estimated at 0.81. If the botanist counted 20 plants with scalloped leaves in the pond, what is the inbreeding coefficient F for this population?

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1
Identify the genotypes associated with each leaf shape: round (r1/r1), arrowhead (r2/r2), and scalloped (r1/r2).
Use the Hardy-Weinberg principle to express the expected genotype frequencies in terms of allele frequencies: p^2 for r1/r1, 2pq for r1/r2, and q^2 for r2/r2, where p is the frequency of r1 and q is the frequency of r2.
Given the frequency of the r1 allele (p = 0.81), calculate the frequency of the r2 allele (q = 1 - p).
Calculate the expected frequency of the heterozygous genotype (r1/r2) using 2pq.
Use the observed number of scalloped plants (20 out of 125) to find the observed frequency of the heterozygous genotype and compare it to the expected frequency to calculate the inbreeding coefficient F.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alleles and Genotypes

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. In this case, the botanist identified two alleles, r1 and r2, which determine the leaf shapes of the water lilies. The genotype of an individual, which is the combination of alleles it possesses, influences its phenotype, or observable traits. Understanding how these alleles interact is crucial for analyzing genetic variation in the population.
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Homozygosity and Heterozygosity

Homozygosity occurs when an individual has two identical alleles at a locus, while heterozygosity refers to having two different alleles. In the observed population, round-leaf plants are homozygous for allele r1, and arrowhead plants are homozygous for allele r2. Scalloped-leaf plants are heterozygous, possessing both alleles. This distinction is important for calculating genetic diversity and understanding the inbreeding coefficient.
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Inbreeding Coefficient (F)

The inbreeding coefficient (F) quantifies the likelihood that two alleles at a locus in an individual are identical by descent, indicating the level of inbreeding in a population. A higher F value suggests increased homozygosity and reduced genetic diversity, which can affect the population's adaptability. To calculate F, one must consider allele frequencies and the observed genotypes, particularly in the context of the scalloped-leaf plants in this study.
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Related Practice
Textbook Question

Achondroplasia is a dominant trait that causes a characteristic form of dwarfism. In a survey of 50,000 births, five infants with achondroplasia were identified. Three of the affected infants had affected parents, while two had normal parents. Calculate the mutation rate for achondroplasia and express the rate as the number of mutant genes per given number of gametes.

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Textbook Question

A recent study examining the mutation rates of 5669 mammalian genes (17,208 sequences) indicates that, contrary to popular belief, mutation rates among lineages with vastly different generation lengths and physiological attributes are remarkably constant [Kumar, S., and Subramanian, S. (2002). Proc. Natl. Acad. Sci. USA 99:803–808]. The average rate is estimated at 12.2×10⁻⁹ per bp per year. What is the significance of this finding in terms of mammalian evolution?

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Textbook Question

What are considered significant factors in maintaining the surprisingly high levels of genetic variation in natural populations?

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Textbook Question

A farmer plants transgenic Bt corn that is genetically modified to produce its own insecticide. Of the corn borer larvae feeding on these Bt crop plants, only 10 percent survive unless they have at least one copy of the dominant resistance allele B that confers resistance to the Bt insecticide. When the farmer first plants Bt corn, the frequency of the B resistance allele in the corn borer population is 0.02. What will be the frequency of the resistance allele after one generation of corn borers have fed on Bt corn?

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Textbook Question

In an isolated population of 50 desert bighorn sheep, a mutant recessive allele c when homozygous causes curled coats in both males and females. The normal dominant allele C produces straight coats. A biologist studying these sheep counts four with curled coats. She also takes blood samples from the population for DNA analysis, which reveals that 17 of the sheep are heterozygous carriers of the c allele. What is the inbreeding coefficient F for this population?

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Textbook Question

To increase genetic diversity in the bighorn sheep population described in Problem 23, ten sheep are introduced from a population where the c allele is absent. Assuming that random mating occurs between the original and the introduced sheep, and that the c allele is selectively neutral, what will be the frequency of c in the next generation?

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