Textbook Question
Describe how populations with substantial genetic differences can form. What is the role of natural selection?
Ch. 26 - Population and Evolutionary Genetics
Klug12th EditionConcepts of Genetics ISBN: 9780135564776
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Table of contents
- Ch. 1 - Introduction to Genetics17
- Ch. 2 - Mitosis and Meiosis36
- Ch. 3 - Mendelian Genetics51
- Ch. 4 - Extensions of Mendelian Genetics74
- Ch. 5 - Chromosome Mapping in Eukaryotes51
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages46
- Ch. 7 - Sex Determination and Sex Chromosomes33
- Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement40
- Ch. 9 - Extranuclear Inheritance31
- Ch. 10 - DNA Structure and Analysis43
- Ch. 11 - DNA Replication and Recombination44
- Ch. 12 - DNA Organization in Chromosomes30
- Ch. 13 - The Genetic Code and Transcription54
- Ch. 14 - Translation and Proteins47
- Ch. 15 - Gene Mutation, DNA Repair, and Transposition41
- Ch. 16 - Regulation of Gene Expression in Bacteria29
- Ch. 17 - Transcriptional Regulation in Eukaryotes41
- Ch. 18 - Post-transcriptional Regulation in Eukaryotes33
- Ch. 19 - Epigenetics33
- Ch. 20 - Recombinant DNA Technology44
- Ch. 21 - Genomic Analysis36
- Ch. 22 - Applications of Genetic Engineering and Biotechnology36
- Ch. 23 - Developmental Genetics37
- Ch. 24 - Cancer Genetics34
- Ch. 25 - Quantitative Genetics and Multifactorial Traits54
- Ch. 26 - Population and Evolutionary Genetics45
Chapter 26, Problem 13
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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Identify the key parameters given: the frequency of affected individuals (q^2) is 0.0001, and the carrier frequency (2pq) is 0.02 in the general population. Here, q is the frequency of the recessive allele, and p is the frequency of the normal allele.
Calculate the allele frequencies p and q using the Hardy-Weinberg equilibrium equations: \(q^2 = 0.0001\) and \(2pq = 0.02\). From \(q^2\), find \(q = \sqrt{0.0001}\), then use \(2pq = 0.02\) to solve for \(p\).
Determine the probability that first cousins both carry the recessive allele inherited from a common ancestor. Since first cousins share about 1/8 of their genes identical by descent, calculate the increased probability that both are carriers due to consanguinity.
Calculate the probability that two first cousins, both carriers, will have an affected child. For autosomal recessive inheritance, this is \(\frac{1}{4}\) if both parents are carriers.
Compare the probability of affected offspring from first cousin marriage to the baseline population risk (0.0001) by multiplying the increased carrier probability by \(\frac{1}{4}\) and then comparing this value to the population frequency.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Autosomal Recessive Inheritance
Autosomal recessive disorders require two copies of a mutant allele for the disease to manifest. Carriers have one normal and one mutant allele but are typically unaffected. When both parents are carriers, there is a 25% chance their child will inherit the disorder. Understanding this inheritance pattern is essential for calculating disease risk in offspring.
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Autosomal Pedigrees
Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle relates allele frequencies to genotype frequencies in a population at equilibrium. It allows estimation of carrier frequency (2pq) and affected frequency (q²) from disease prevalence. This concept helps interpret the given population data and estimate probabilities of genotypes in the general population.
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Consanguinity and Increased Genetic Risk
Consanguineous marriages, such as between first cousins, increase the probability that both parents share alleles inherited from a common ancestor. This raises the chance of offspring inheriting two copies of a recessive allele, increasing disease risk compared to unrelated parents. Calculating this increased risk requires understanding pedigree relationships and inbreeding coefficients.
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Related Practice
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.
Textbook Question
If the initial allele frequencies are p = 0.5 and q = 0.5 and allele a is a lethal recessive, what will be the frequencies after 1, 5, 10, 25, 100, and 1000 generations?
Textbook Question
Under what circumstances might a lethal dominant allele persist in a population?
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Textbook Question
Consider a population in which the frequency of allele A is p = 0.7 and the frequency of allele a is q = 0.3 and where the alleles are codominant. What will be the allele frequencies after one generation if the following occurs?
wAA = 0.8, wAa = 1, waa = 0.8
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.