Figure 19-9 shows the trait distributions before and after a cycle of artificial selection. Does the variance of the trait appear to have changed as a result of selection? Explain.

Figure 19-11 shows the expected distributions for the three genotypic classes if the B locus is a QTL affecting the trait value.

Figure 19-16 shows the results of a QTL fine-mapping experiment. Which gene would be implicated as controlling fruit weight if the mean fruit weight for each line was as follows?

Figure 19-17 shows a set of haplotypes. Suppose these are haplotypes for a chromosomal segment from 18 haploid yeast strains. On the right edge of the figure, the S and D indicate whether the strain survives (S) or dies (D) at high temperature (40°C). Using the χ² test (see Chapter 3) and Table 3-1, does either SNP1 or SNP6 show evidence for an association with the growth phenotype? Explain.

Figure 19-18a shows a plot of P values (represented by the dots) along the chromosomes of the dog genome. Each P value is the result of a statistical test of association between a SNP and body size. Other than the cluster of small P values near IGF1, do you see any chromosomal regions with evidence for a significant association between a SNP and body size? Explain.

Figure 19-19 shows plots of P values (represented by the dots) along the chromosomes of the human genome. Each P value is the result of a statistical test of association between a SNP and a disease condition. There is a cluster, or spike, of statistically significant P values (green dots) at the gene HLA-DRB1 for two diseases. Why might this particular gene contribute to susceptibility for the autoimmune diseases rheumatoid arthritis and type 1 diabetes?

Distinguish between continuous and discontinuous variation in a population, and give some examples of each.

What are the central assumptions of the multifactorial hypothesis?

The table below shows a distribution of bristle number in a Drosophila population. Calculate the mean, variance, and standard deviation for these data.

Suppose that the mean IQ in the United States is roughly 100 and the standard deviation is 15 points. People with IQs of 145 or higher are considered “geniuses” on some scales of measurement. What percentage of the population is expected to have an IQ of 145 or higher? In a country with 300 million people, how many geniuses are there expected to be?

In a sample of adult women from the United States, the average height was 164.4 cm and the standard deviation was 6.2 cm. Women who are more than 2 standard deviations above the mean are considered very tall, and women who are more than 2 standard deviations below the mean are considered very short. Height in women is normally distributed.

A bean breeder is working with a population in which the mean number of pods per plant is 50 and the variance is 10 pods². The broad-sense heritability is known to be 0.8. Given this information, can the breeder be assured that the population will respond to selection for an increase in the number of pods per plant in the next generation?

The table below shows the number of piglets per litter for a group of 60 sows. What is the mean number of piglets per litter? What is the relative frequency of litters with at least 12 piglets?

A chicken breeder is working with a population in which the mean number of eggs laid per hen in one month is 28 and the variance is 5 eggs². The narrow-sense heritability is known to be 0.8. Given this information, can the breeder expect that the population will respond to selection for an increase in the number of eggs per hen in the next generation?

The narrow-sense heritability of the number of peas per pod in a population of sugar snap peas is 0.5. The mean of the population is 6.2 peas per pod. A plant breeder selects one plant with 6.8 peas per pod and crosses with a second plant that has 8.0 peas per pod. What is the expected numbers of peas per pod among the offspring of this cross?

QTL mapping and GWA (association) mapping are two different methods used to identify genes that affect complex traits. For each of the following statements, choose whether it applies to QTL mapping, association mapping, or both.

758

In a large herd of cattle, three different characters showing continuous distribution are measured, and the variances in the following table are calculated:

In a species of the Darwin’s finches (Geospiza fortis), the narrow-sense heritability of bill depth has been estimated to be 0.79. Bill depth is correlated with the ability of the finches to eat large seeds. The mean bill depth for the population is 9.6 mm. A male with a bill depth of 10.8 mm is mated with a female with a bill depth of 9.8 mm. What is the expected value for bill depth for the offspring of this mating pair?

Two inbred lines of laboratory mice are intercrossed. In the F₁ (which have identical genotypes at all loci), the variance in adult weight is measured at 3 g². The F₁ animals are intercrossed to create an F₂ in which the variance in adult weight is 16 g². Estimate the broad heritability of adult weight in the F₂ population of this experiment. (The environments in which the F₁ and F₂ animals were reared were equivalent.)

The table below shows the weights of 100 individual mice of the same inbred strain reared on different diets. For an individual mouse that weights 27 g, how much of its weight is due to its genetics and how much to the specific diet it was fed (environment)? (Other than diet, the mice were reared in equivalent environments.)

The table below contains measurements of total serum cholesterol (mg/dl) for 10 sets of monozygotic twins who were reared apart. Calculate the following: overall mean, overall variance, covariance between the twins, and broad-sense heritability (H²).

The table below contains the height in centimeters for 10 sets of adult women twins. Calculate the correlation coefficient (r) between the heights of the sisters for the twin pairs.

Population A consists of 100 hens that are fully isogenic and that are reared in a uniform environment. The average weight of the eggs they lay is 52 g, and the variance is 3.5 g². Population B consists of 100 genetically variable hens that produce eggs with a mean weight of 52 g and a variance of 21.0 g². Population B is raised in an environment that is equivalent to that of Population A. What is the environmental variance (V_e) for egg weight? What is the genetic variance in Population B? What is the broad-sense heritability in Population B?

Maize plants in a population are on average 180 cm tall. Narrow-sense heritability for plant height in this population is 0.5. A breeder selects plants that are 10 cm taller on average than the population mean to produce the next generation, and the breeder continues applying this level of selection for eight generations. What will be the average height of the plants after eight generations of selection? Assume that h² remains 0.5 and V_e does not change over the course of the experiment.

In a population of Drosophila melanogaster reared in the laboratory, the mean wing length is 0.55 mm and the range is 0.35 to 0.65. A geneticist selects a female with wings that are 0.42 mm in length and mates her with a male that has wings that are 0.56 mm in length.

Different species of crickets have distinct songs, and they use these songs for mate recognition. Researchers crossed two species of Hawaiian crickets (Laupala paranigra and L. kohalensis) whose songs are distinguished by pulse rate (the number of pulses per second; Shaw et al., Molecular Ecology 16, 2007, 2879–2892.) Then, they mapped QTL in the F₂ population derived from this cross. Six autosomal QTL were detected. The mean trait values (pulses per second) at the three genotypic classes in the F₂ for each QTL are shown in the table below, where P indicates the L. paranigra allele and K indicates the L. kohalensis allele.

Question 26 refers to QTL on the cricket autosomes. For the sex chromosomes, females crickets are XX and males crickets are XO, having just one X chromosome but no Y chromosome. Can QTL for pulse rate be mapped on cricket X chromosomes? If the song is only sung by male crickets, can the dominance effects of QTL on the X be estimated?

GWA studies reveal statistical correlations between the genotypes at marker loci in genes and complex traits. Do GWA studies prove that allelic variation in a gene actually causes the variation in the trait? If not, what experiments could prove that allelic variants in a gene in a population are responsible for variation in a trait?

The ocular albinism-2 (OCA2) gene and the melanocortin-1-receptor (MC1R) gene are both involved in melanin metabolism in skin cells in humans. To test whether variation at these genes contributes to sun sensitivity and the associated risk of being afflicted with skin cancer, you perform association analyses. A sample of 1000 people from Iceland were asked to classify themselves as having tanning or burning (nontanning) skin when exposed to the sun. The individuals were also genotyped for a SNP in each gene (rs7495174 and rs1805007). The table shows the number of individuals in each class.