Chapter Questions
The following data for the $M$ -N blood types were obtained from native villages in Central and North America:$$\begin{array}{lcccc}\text { Group } & \text { Sample Size } & \mathbf{M} & \mathbf{M N} & \mathbf{N} \\\hline \text { Central American } & 86 & 53 & 29 & 4 \\\text { North American } & 278 & 78 & 61 & 139 \\\hline\end{array}$$Calculate the frequencies of the $L^{M}$ and $L^{N}$ alleles for the two groups.
The frequency of an allele in a large randomly mating population is $0.2 .$ What is the frequency of heterozygous carriers?
The incidence of recessive albinism is 0.0004 in a human population. If mating for this trait is random in the population, what is the frequency of the recessive allele?
In a sample from an African population, the frequencies of the $L^{M}$ and $L^{N}$ alleles were 0.78 and $0.22,$ respectively. If the population mates randomly with respect to the M-N blood types, what are the expected frequencies of the $M, M N,$ and $N$ phenotypes?
Human beings carrying the dominant allele $T$ can taste the substance phenylthiocarbamide (PTC). In a population in which the frequency of this allele is $0.4,$ what is the probability that a particular taster is homozygous?
A gene has three alleles, $A_{1}, A_{2},$ and $A_{3},$ with frequencies $0.6,0.3,$ and $0.1,$ respectively. If mating is random, predict the combined frequency of all the heterozygotes in the population.
Hemophilia is caused by an X-linked recessive allele. In a particular population, the frequency of males with hemophilia is $1 / 4000 .$ What is the expected frequency of females with hemophilia?
In Drosophila the ruby eye phenotype is caused by a recessive, X-linked mutant allele. The wild-type eye color is red. A laboratory population of Drosopbila is started with 25 percent ruby-eyed females, 25 percent homozygous red-eyed females, 5 percent ruby-eyed males, and 45 percent red-eyed males. (a) If this population mates randomly for one generation, what is the expected frequency of ruby-eyed males and females?(b) What is the frequency of the recessive allele in each of the sexes?
A trait determined by an X-linked dominant allele shows 100 percent penetrance and is expressed in 36 percent of the females in a population. Assuming that the population is in Hardy-Weinberg equilibrium, what proportion of the males in this population express the trait?
A phenotypically normal couple has had one normal child and a child with cystic fibrosis, an autosomal recessive disease. The incidence of cystic fibrosis in the population from which this couple came is $1 / 500$. If their normal child eventually marries a phenotypically normal person from the same population, what is the risk that the newlyweds will produce a child with cystic fibrosis?
What frequencies of alleles $A$ and $a$ in a randomly mating population maximize the frequency of heterozygotes?
In an isolated population, the frequencies of the $I^{A}, I^{B}$ and $i$ alleles of the $A-B-O$ blood type gene are, respectively, $0.15,0.25,$ and $0.60 .$ If the genotypes of the $\mathrm{A}-\mathrm{B}-\mathrm{O}$ blood type gene are in Hardy-Weinberg proportions, what fraction of the people who have type $A$ blood in this population are expected to be homozygous for the $F^{A}$ allele?
In a survey of moths collected from a natural population, a researcher found 51 dark specimens and 49 light specimens. The dark moths carry a dominant allele, and the light moths are homozygous for a recessive allele. If the population is in Hardy-Weinberg equilibrium, what is the estimated frequency of the recessive allele in the population? How many of the dark moths in the sample are likely to be homozygous for the dominant allele?
A population of Hawaiian Drosopbila is segregating two alleles, $P^{1}$ and $P^{2}$, of the phosphoglucose isomerase $(P G I)$ gene. In a sample of 100 flies from this population, 30 were $P^{1} P^{1}$ homozygotes, 60 were $P^{1} P^{2}$ heterozygotes, and 10 were $P^{2} P^{2}$ homozygotes.(a) What are the frequencies of the $P^{1}$ and $P^{2}$ alleles in this sample?(b) Perform a chisquare test to determine if the genotypes in the sample are in Hardy-Weinberg proportions.(c) Assuming that the sample is representative of the population, how many generations of random mating would be required to establish Hardy-Weinberg proportions in the population?
In a large population that reproduces by random mating, the frequencies of the genotypes $G G, G g,$ and $g g$ are 0.04 $0.32,$ and $0.64,$ respectively. Assume that a change in the climate induces the population to reproduce exclusively by self-fertilization. Predict the frequencies of the genotypes in this population after many generations of selffertilization.
The frequencies of the alleles $A$ and $a$ are 0.6 and $0.4,$ respectively, in a particular plant population. After many generations of random mating, the population goes through one cycle of self-fertilization. What is the expected frequency of heterozygotes in the progeny of the self-fertilized plants?
Each of two isolated populations is in Hardy-Weinberg equilibrium with the following genotype frequencies:$$\begin{array}{llll}\text { Genotype: } & A A & A a & a a \\\text { Frequency in Population 1: } & 0.04 & 0.32 & 0.64 \\\text { Frequency in Population 2: } & 0.64 & 0.32 & 0.04\end{array}$$(a) If the populations are equal in size and they merge to form a single large population, predict the allele and genotype frequencies in the large population immediately after merger.(b) If the merged population reproduces by random mating, predict the genotype frequencies in the next generation.(c) If the merged population continues to reproduce by random mating, will these genotype frequencies remain constant?
A population consists of 25 percent tall individuals $\text { (genotype }T T), 25 \text { percent short individuals (genotype } t t)$ and 50 percent individuals of intermediate height (genotype $T t$ ). Predict the ultimate phenotypic and genotypic composition of the population if, generation after generation, mating is strictly assortative (that is, tall individuals mate with tall individuals, short individuals mate with short individuals, and intermediate individuals mate with intermediate individuals).
In controlled experiments with different genotypes of an insect, a researcher has measured the probability of survival from fertilized eggs to mature, breeding adults. The survival probabilities of the three genotypes tested are: $0.92(\text { for } G G), 0.90 \text { (for } G g),$ and 0.56 (for $g g$ ). If all breeding adults are equally fertile, what are the relative fitnesses of the three genotypes? What are the selection coefficients for the two least fit genotypes?
In a large randomly mating population, 0.84 of the individuals express the phenotype of the dominant allele $A$ and 0.16 express the phenotype of the recessive allele $a$(a) What is the frequency of the dominant allele?(b) If the $a a$ homozygotes are 5 percent less fit than the other two genotypes, what will the frequency of $A$ be in the next generation?
Because individuals with cystic fibrosis die before they can reproduce, the coefficient of selection against them is $s=1 .$ Assume that heterozygous carriers of the recessive mutant allele responsible for this disease are as fit as wild-type homozygotes and that the population frequency of the mutant allele is 0.02(a) Predict the incidence of cystic fibrosis in the population after one generation of selection.(b) Explain why the incidence of cystic fibrosis hardly changes even with $s=1$
For each set of relative fitnesses for the genotypes $A A$ $A a,$ and $a a,$ explain how selection is operating. Assume that $0<t<s<1$$$\begin{array}{llll} & A A & A a & a a \\\hline \text { Case 1 } & 1 & 1 & 1-s \\\text { Case 2 } & 1-s & 1-s & 1 \\\text { Case 3 } & 1 & 1-t & 1-s \\\text { Case 4 } & 1-s & 1 & 1-t \\\hline\end{array}$$
The frequency of newborn infants homozygous for a recessive lethal allele is about 1 in 25,000 . What is the expected frequency of carriers of this allele in the population?
A population of size 50 reproduces in such a way that the population size remains constant. If mating is random, how rapidly will genetic variability, as measured by the frequency of heterozygotes, be lost from this population?
A population is segregating three alleles, $A_{1}, A_{2},$ and $A_{3}$ with frequencies $0.2,0.5,$ and $0.3,$ respectively. If these alleles are selectively neutral, what is the probability that $A_{2}$ will ultimately be fixed by genetic drift? What is the probability that $A_{3}$ will ultimately be lost by genetic drift?
A small island population of mice consists of roughly equal numbers of males and females. The Y chromosome in one-fourth of the males is twice as long as the $Y$ chromosome in the other males because of an expansion of heterochromatin. If mice with the large $Y$ chromosome have the same fitness as mice with the small $Y$ chromosome, what is the probability that the large $Y$ chromosome will ultimately be fixed in the mouse population?
In some regions of West Africa, the frequency of the $H B B^{S}$ allele is $0.2 .$ If this frequency is the result of a dynamic equilibrium due to the superior fitness of $H B B^{S} H B B^{A}$ heterozygotes, and if $H B B^{S} H B B^{S}$ homozygotes are essentially lethal, what is the intensity of selection against the $H B B^{A} H B B^{A}$ homozygotes?
Mice with the genotype $H b$ are twice as fit as either of the homozygotes $H H$ and $b b$. With random mating, what is the expected frequency of the $b$ allele when the mouse population reaches a dynamic equilibrium because of balancing selection?
A completely recessive allele $g$ is lethal in homozygous condition. If the dominant allele $G$ mutates to $g$ at a rate of $10^{-6}$ per generation, what is the expected frequency of the lethal allele when the population reaches mutationselection equilibrium?
Individuals with the genotype $b b$ are 20 percent less fit than individuals with the genotypes $B B$ or $B b$. If $B$ mutates to $b$ at a rate of $10^{-6}$ per generation, what is the expected frequency of the allele $b$ when the population reaches mutation-selection equilibrium?