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Ch. 26 - Population and Evolutionary Genetics
Chapter 25, 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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span>Identify the total number of births surveyed, which is 50,000.</span
span>Determine the number of infants with achondroplasia, which is 5.</span
span>Out of these 5 infants, 2 had normal parents, indicating new mutations.</span
span>Since achondroplasia is a dominant trait, each affected individual must have one mutant allele. Therefore, the 2 infants with normal parents represent 2 new mutations.</span
span>Calculate the mutation rate by dividing the number of new mutations (2) by the total number of gametes, which is twice the number of births (100,000), since each birth involves two gametes (one from each parent).</span

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Key Concepts

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

Dominant and Recessive Traits

In genetics, traits can be classified as dominant or recessive based on their inheritance patterns. A dominant trait, like achondroplasia, only requires one copy of the mutant allele to be expressed in the phenotype. This means that if an individual inherits the dominant allele from one parent, they will exhibit the trait, regardless of the other allele inherited.
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Variations on Dominance

Mutation Rate

The mutation rate refers to the frequency at which new mutations occur in a given gene or organism over a specific time period. It is often expressed as the number of new mutations per generation or per gamete. In the context of achondroplasia, calculating the mutation rate involves determining how many new cases arise in relation to the total number of gametes produced.
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Mutations and Phenotypes

Gametes and Genetic Transmission

Gametes are the reproductive cells (sperm and eggs) that carry genetic information from one generation to the next. Each gamete contains half the genetic material of an individual, and during fertilization, they combine to form a zygote. Understanding the role of gametes is crucial for calculating mutation rates, as it helps in determining how many opportunities exist for mutations to be passed on to offspring.
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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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