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Ch. 26 - Population and Evolutionary Genetics
Klug - Concepts of Genetics  12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
Chapter 26, Problem 16

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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Identify the key information: Achondroplasia is a dominant trait, 50,000 births were surveyed, 5 infants had achondroplasia, 3 had affected parents, and 2 had normal parents.
Understand that affected infants with normal parents represent new mutations, since achondroplasia is dominant and the parents are unaffected (homozygous normal).
Calculate the total number of gametes that contributed to the 50,000 births. Since each individual has two parents, the total number of gametes is twice the number of births: \$2 \times 50,000$.
Determine the number of new mutations, which is equal to the number of affected infants with normal parents (2 in this case).
Calculate the mutation rate by dividing the number of new mutations by the total number of gametes: mutation rate = \(\frac{\text{number of new mutations}}{\text{total number of gametes}} = \frac{2}{2 \times 50,000}\).

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Key Concepts

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

Dominant Inheritance

Dominant inheritance means that only one copy of a mutant allele is needed for the trait to be expressed. In achondroplasia, individuals with one mutant gene show the dwarfism phenotype, so affected parents can pass the trait directly to offspring. Understanding dominance helps interpret how traits appear in families.
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04:37
Variations on Dominance

Mutation Rate Calculation

Mutation rate is the frequency at which new mutations arise in a gene per generation. It can be estimated by comparing the number of affected individuals with no affected parents (new mutations) to the total number of gametes in the population. This rate is often expressed as mutations per number of gametes.
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Mutations and Phenotypes

Gametes and Population Genetics

Gametes are reproductive cells (sperm and eggs) that carry genes to the next generation. In population genetics, the total number of gametes is twice the number of individuals because each individual produces two gametes. This concept is essential for calculating mutation rates in a population.
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Gamete Development
Related Practice
Textbook Question

Assume that a recessive autosomal disorder occurs in 1 of 10,000 individuals (0.0001) in the general population and that in this population about 2 percent (0.02) of the individuals are carriers for the disorder. Estimate the probability of this disorder occurring in the offspring of a marriage between first cousins. Compare this probability to the population at large.

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Textbook Question

One of the first Mendelian traits identified in humans was a dominant condition known as brachydactyly. This gene causes an abnormal shortening of the fingers or toes (or both). At the time, some researchers thought that the dominant trait would spread until 75 percent of the population would be affected (because the phenotypic ratio of dominant to recessive is 3 : 1). Show that the reasoning was incorrect.

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Textbook Question

Describe how populations with substantial genetic differences can form. What is the role of natural selection?

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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 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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