Chapter Questions
If heart disease is considered to be a threshold trait, what genetic and environmental factors might contribute to the underlying liability for a person to develop this disease?
A wheat variety with red kernels (genotype $A^{\prime} A^{\prime}$ $\left.B^{\prime} B^{\prime}\right)$ was crossed with a variety with white kernels (genotype $A A B B$ ). The $\mathrm{F}_{1}$ were intercrossed to produce an $\mathrm{F}_{2}$ If each primed allele increases the amount of pigment in the kernel by an equal amount, what phenotypes will be expected in the $\mathrm{F}_{2}$ ? Assuming that the $A$ and $B$ loci assort independently, what will the phenotypic frequencies be?
For alcoholism, the concordance rate for monozygotic twins is 55 percent, whereas for dizygotic twins, it is 28 percent. Do these data suggest that alcoholism has a genetic basis?
The height of the seed head in wheat at maturity is determined by several genes. In one variety, the head is just 9 inches above the ground; in another, it is 33 inches above the ground. Plants from the 9 -inch variety were crossed to plants from the 33 -inch variety. Among the $\mathrm{F}_{1},$ the seed head was 21 inches above the ground. After self-fertilization, the $F_{1}$ plants produced an $F_{2}$ population in which 9 -inch and 33 -inch plants each appeared with a frequency of $1 / 256 .$ (a) How many genes are involved in the determination of seed head height in these strains of wheat?(b) How much does each allele of these genes contribute to seed head height?(c) If a 21 -inch $\mathrm{F}_{1}$ plant were crossed to a 9 -inch plant, how often would you expect 18-inch wheat to occur in the progeny?
Assume that size in rabbits is determined by genes with equal and additive effects. From a total of $2012 \mathrm{F}_{2}$ progeny from crosses between true-breeding large and small varieties, eight rabbits were as small as the small variety and eight were as large as the large variety. How many sizedetermining genes were segregating in these crosses?
A sample of 20 plants from a population was measured in inches as follows: 18,21,20,23,20,21,20,22,19 $20,17,21,20,22,20,21,20,22,19,$ and $23 .$ Calculate(a) the mean,(b) the variance, and(c) the standard deviation.
Quantitative geneticists use the variance as a measure of scatter in a sample of data; they calculate this statistic by averaging the squared deviations between each measurement and the sample mean. Why don't they simply measure the scatter by computing the average of the deviations without bothering to square them?
Two inbred strains of corn were crossed to produce an $\mathrm{F}_{1},$ which was then intercrossed to produce an $\mathrm{F}_{2} .$ Data on ear length from a sample of $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ individuals gave phenotypic variances of $15.2 \mathrm{cm}^{2}$ and $27.6 \mathrm{cm}^{2},$ respectively. Why was the phenotypic variance greater for the $\mathrm{F}_{1},$ than for the $\mathrm{F}_{2} ?$
A study of quantitative variation for abdominal bristle number in female Drosopbila yielded estimates of $V_{T}=$ $6.08, V_{g}=3.17,$ and $V_{e}=2.91 .$ What was the broad-sense heritability?
A researcher has been studying kernel number on ears of corn. In one highly inbred strain, the variance for kernel number is $426 .$ Within this strain, what is the broad-sense heritability for kernel number?
Measurements on ear length were obtained from three populations of corn-two inbred varieties and a randomly pollinated population derived from a cross between the two inbred strains. The phenotypic variances were $9.2 \mathrm{cm}^{2}$ and $9.6 \mathrm{cm}^{2}$ for the two inbred varieties and $26.4 \mathrm{cm}^{2}$ for the randomly pollinated population. Estimate the broad-sense heritability of ear length for these populations.
Figure 22.4 summarizes data on maturation time in populations of wheat. Do these data provide any insight as to whether or not this trait is influenced by dominance? Explain.
A person claims that the narrow-sense heritability for body mass in human beings is $0.7,$ while the broad-sense heritability is only $0.3 .$ Why must there be an error?
The mean value of a trait is 100 units, and the narrowsense heritability is $0.4 .$ A male and a female measuring 124 and 126 units, respectively, mate and produce a large number of offspring, which are reared in an average environment. What is the expected value of the trait among these offspring?
The narrow-sense heritability for abdominal bristle number in a population of Drosopbila is $0.3 .$ The mean bristle number is $12 .$ A male with 10 bristles is mated to a female with 20 bristles, and a large number of progeny are scored for bristle number. What is the expected mean number of bristles among these progeny?
A breeder is trying to decrease the maturation time in a population of sunflowers. In this population, the mean time to flowering is 100 days. Plants with a mean flowering time of only 90 days were used to produce the next generation. If the narrow-sense heritability for flowering time is $0.2,$ what will the average time to flowering be in the next generation?
A fish breeder wishes to increase the rate of growth in a stock by selecting for increased length at six weeks after hatching. The mean length of six-week-old fingerlings is currently $10 \mathrm{cm} .$ Adult fish that had a mean length of $15 \mathrm{cm}$ at six weeks of age were used to produce a new generation of fingerlings. Among these, the mean length was $12.5 \mathrm{cm} .$ Estimate the narrow-sense heritability of fingerling length at six weeks of age and advise the breeder about the feasibility of the plan to increase growth rate.
Leo's IQ is 86 and Julie's IQ is $110 .$ The mean IQ in the population is $100 .$ Assume that the narrow-sense heritability for IQ is $0.4 .$ What is the expected IQ of Leo and Julie's first child?
One way to estimate a maximum value for the narrowsense heritability is to calculate the correlation between half-siblings that have been reared apart and divide it by the fraction of genes that half-siblings share by virtue of common ancestry. A study of human half-siblings found that the correlation coefficient for height was $0.14 .$ From this result, what is the maximum value of the narrowsense heritability for height in this population?
A selection differential of $40 \mu \mathrm{g}$ per generation was used in an experiment to select for increased pupa weight in Tribolium. The narrow-sense heritability for pupa weight was estimated to be $0.3 .$ If the mean pupa weight was initially $2000 \mu \mathrm{g}$ and selection was practiced for 10 generations, what was the mean pupa weight expected to become?
On the basis of the observed correlations for personality traits shown in Table $22.5,$ what can you say about the value of the environmentality $\left(C^{2} \text { in Table } 22.3\right)$ ?
Correlations between relatives provide estimates of the broad and narrow-sense heritabilties on the assumption that the genetic and environmental factors influencing quantitative traits are independent of each other and that they do not interact in some peculiar way. In Chapter 19 we considered epigenetic modifications of chromatin that regulate genes and noted the possibility that some of these modifications might be induced by environmental factors. How could epigenetic influences on complex traits be incorporated into the basic theory of quantitative genetics?