Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Problems

1. | What are the forces that can change the frequency of an allele in a population? See answer

2. | In a population of mice, there are two alleles of the A locus (A1 and A2). Tests showed that in this population there are 384 mice of genotype A1 / A1, 210 of A1 / A2, and 260 of A2 / A2. What are the frequencies of the two alleles in the population? See answer

3. | In a randomly mating laboratory population of Drosophila, 4 percent of the flies have black bodies (black is the autosomal recessive b) and 96 percent have brown bodies (the normal color B). If this population is assumed to be in Hardy-Weinberg equilibrium, what are the allelic frequencies of B and b and the genotypic frequencies of B / B and B / b?

4. | In a population, the D d mutation rate is 4 × 106 If p = 0.8 today, what will p be after 50,000 generations? See answer

5.

You are studying protein polymorphism in a natural population of a certain species of a sexually reproducing haploid organism. You isolate many strains from various parts of the test area and run extracts from each strain on electrophoretic gels. You stain the gels with a reagent specific for enzyme X and find that in the population there is a total of, say, five electrophoretic variants of enzyme X. You speculate that these variants represent various alleles of the structural gene for enzyme X.
a. How could you demonstrate that this is so, both genetically and biochemically? (You can make crosses, make diploids, run gels, test enzyme activities, test amino acid sequences, and so forth). Outline the steps and conclusions precisely.
b. Name at least one other possible way of generating the different electrophoretic variants, and say how you would distinguish this possibility from the one described here.

6.

A study made in 1958 in the mining town of Ashibetsu in the Hokkaido province of Japan revealed the frequencies of MN blood type genotypes (for individuals and for married couples) shown in the following table:

a. Show whether or not the population is in Hardy-Weinberg equilibrium with respect to the MN blood types.
b. Show whether mating is random with respect to MN blood types.

(Problem 6 is from J. Kuspira and G. W. Walker, Genetics: Questions and Problems. Copyright © 1973 by McGraw-Hill.)

See answer

7.

Consider the populations that have the genotypes shown in the following table:

a. Which of the populations are in Hardy-Weinberg equilibrium?
b. What are p and q in each population?
c. In population 10, it is discovered that the A a mutation rate is 5 × 106 and that reverse mutation is negligible. What must be the fitness of the a / a phenotype?
d. In population 6, the a allele is deleterious; furthermore, the A allele is incompletely dominant, so that A / A is perfectly fit, A / a has a fitness of 0.8, and a / a has a fitness of 0.6. If there is no mutation, what will p and q be in the next generation?
See answer

8.

Colorblindness results from a sex-linked recessive allele. One in every 10 males is colorblind.
a. What proportion of all women are colorblind?
b. By what factor is colorblindness more common in men (or, how many colorblind men are there for each colorblind woman)?
c. In what proportion of marriages would colorblindness affect half the children of each sex?
d. In what proportion of marriages would all children be normal?
e. In a population that is not in equilibrium, the frequency of the allele for colorblindness is 0.2 in women and 0.6 in men. After one generation of random mating, what proportion of the female progeny will be colorblind? What proportion of the male progeny?
f. What will the allelic frequencies be in the male and in the female progeny in part e?

(Problem 8 courtesy of Clayton Person.)

9. | In a wild population of beetles of species X, you notice that there is a 3:1 ratio of shiny to dull wing covers. Does this prove that shiny is dominant? (Assume that the two states are caused by the alleles of one gene.) If not, what does it prove? How would you elucidate the situation? See answer

10. | It seems clear that most new mutations are deleterious. Why?

11. | Most mutations are recessive to the wild type. Of those rare mutations that are dominant in Drosophila, for example, the majority turn out either to be chromosomal aberrations or to be inseparable from chromosomal aberrations. Can you explain why the wild type is usually dominant? See answer

12. | Ten percent of the males of a large and randomly mating population are colorblind. A representative group of 1000 from this population migrates to a South Pacific island, where there are already 1000 inhabitants and where 30 percent of the males are colorblind. Assuming that Hardy-Weinberg equilibrium applies throughout (in the two original populations before emigration and in the mixed population immediately after immigration), what fraction of males and females can be expected to be colorblind in the generation immediately after the arrival of the immigrants?

13. | Using pedigree diagrams, find the probability of homozygosity by descent of the offspring of (a) parentoffspring matings; (b) first-cousin matings; (c) auntnephew or uncleniece matings.

14. | In a survey of Native American tribes in Arizona and New Mexico, it was found that albinos were completely absent or very rare in most groups (there is one albino per 20,000 North American Caucasians). However, in three Native American populations, albino frequencies are exceptionally high: one per 277 Native Americans in Arizona; one per 140 Jemez in New Mexico; and one per 247 Zuni in New Mexico. All three of these populations are culturally but not linguistically related. What possible factors might explain the high incidence of albinos in these three tribes? See answer

15. | In an animal population, 20 percent of the individuals are A / A, 60 percent are A / a, and 20 percent are a / a. What are the allelic frequencies? In this population, mating is always with like phenotype but is random within phenotype. What genotypic and allelic frequencies will prevail in the next generation? Such assortative mating is common in animal populations. Another type of assortative mating is that which occurs only between unlike phenotypes: answer the preceding question with this restriction imposed. What will the end result be after many generations of mating of both types?

16. | In Drosophila, a stock isolated from nature has an average of 36 abdominal bristles. By the selective breeding of only those flies with more bristles, the mean is raised to 56 in 20 generations. What is the source of this genetic flexibility? The 56-bristle stock is infertile, so selection is relaxed for several generations and the bristle number drops to about 45. Why doesn't it drop back to 36? When selection is reapplied, 56 bristles are soon attained, but this time the stock is not sterile. How can this situation arise?

17. | The fitnesses of three genotypes are W A/A = 0.9,W A/a = 1.0, andW a/a = 0.7. a. If the population starts at the allelic frequency p = 0.5, what is the value of p in the next generation? b. What is the predicted equilibrium allelic frequency? See answer

18. | A / A and A / a individuals are equally fertile. If 0.1 percent of the population is a / a, what selection pressure exists against a / a if the A a mutation rate is 105? See answer

19. | Gene B is a deleterious autosomal dominant. The frequency of affected individuals is 4.0 × 106 The reproductive capacity of these individuals is about 30 percent that of normal individuals. Estimate m , the rate at which b mutates to its deleterious allele B.

20. | Of 31 children born of fatherdaughter matings, 6 died in infancy, 12 were very abnormal and died in childhood, and 13 were normal. From this information, calculate roughly how many recessive lethal genes we have, on average, in our human genomes. For example, if the answer is 1, then a daughter would stand a 50 percent chance of carrying the gene, and the probability of the union's producing a lethal combination would be 1/2 × 1/4 = 1/8. (So obviously 1 is not the answer.) Consider also the possibility of undetected fatalities in utero in such matings. How would they affect your result? See answer

21.

If we define the total selection cost to a population of deleterious recessive genes as the loss of fitness per individual affected (s) multiplied by the frequency of affected individuals (q2), then

a. Suppose that a population is at equilibrium between mutation and selection for a deleterious recessive gene, where s = 0.5 and µ = 105 What is the equilibrium frequency of the gene? What is the genetic cost?
b. Suppose that we start irradiating individual members of the population, so that the mutation rate doubles. What is the new equilibrium frequency of the gene? What is the new genetic cost?
c. If we do not change the mutation rate but we lower the selection intensity to s = 0.3 instead, what happens to the equilibrium frequency and the genetic cost?

© 1999 by W. H. Freeman and Company. All rights reserved.