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Quantitative Genetics Practice Test (page 2)

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By — McGraw-Hill Professional
Updated on Aug 23, 2011

Polygenic Traits Questions

  1. Beginning at some arbitrary date, two varieties of wheat were scored for the length of time (in days) to heading, from which the following means were obtained: variety X = 13.0 days, variety Y = 27.6 days. From a survey of 5,504,000 F2 progeny, 86 were found to head out in 13 days or less. How many pairs of factors are probably contributing to early flowering?
  2. Suppose that the average skin color on one racial population is 0.43 (measured by the reflectance of skin to red light of 685-nm wavelength); the average skin color of a population racially distinct from the first is 0.23; and racial hybrids between these two populations average 0.33. If about 1/150 offspring from hybrid (racially mixed) parents have skin colors as extreme as the average of either race, estimate the number of segregating loci in the hybrid parents that contribute to skin color variability in their offspring.

The Normal Distribution Questions

  1. From a sample of 10 pig body weights determine     (a)  mean body weight,     (b)  sample standard deviation (s),     (c)  the weight that will probably be exceeded by 2%of this population. Pig weights: 210, 215, 220, 225, 215, 205, 220, 210, 215, 225.
  2. Suppose six pairs of genes were contributing to a quantitative trait in a cultivated crop. Two parental lines with averages of 13,000 lb/acre and 7000 lb/acre produced an intermediate hybrid F1 with a variance of 250,000 lb2. Estimate the standard deviation of the F2 by formula (8.6).
  3. Two strains of mice were tested for susceptibility to a carcinogenic drug. The susceptible strain had an average of 75.4 tumorous lung nodules, whereas the resistant strain failed to develop nodules. The F1 from crossing these two strains had an average of 12.5 nodules with a standard deviation of ±5.3; the F2 had 10.0 ± 14.1 nodules. Estimate the number of gene pairs contributing to tumor susceptibility by use of formula (8.6).

Types of Gene Action Questions

  1. Calculate the metric values of the parents and their F1 hybrids in the cross AA B'B' CC D'D' × A'A' BB C'C' DD assuming     (a)  additive gene action where unprimed alleles contribute 3 units each to the phenotype and primed alleles contribute 6 units each,     (b)  primed alleles are fully dominant to unprimed alleles; at a given locus, genotypes with one or two primed alleles produce 12 units and the recessive genotype produces 6 units.

Heritability Questions

  1. Let Vi = phenotypic variance between identical twins, Vf = phenotypic variance between fraternal twins, and heritability (h2) = (VfVi)/Vf. Given the following differences in intelligence quotients (IQ) of 20 pairs of twins (all females, reared together, and identically tested at the same age), estimate the heritability of IQ.
  2. Supplementary Problems

  3. Suppose that population A has a mean IQ of 85 and that of population B is 100. Estimates of heritability of IQ in both populations are relatively high (0.4 to 0.8). Explain why each of the following statements is false.
  4. (a)  Heritability estimates measure the degree to which a trait is determined by genes.

    (b)  Since the heritability of IQ is relatively high, the average differences between the two populations must be largely due to genetic differences.

    (c)  Since population B has a higher average IQ than population A, population B is genetically superior to A.

  5. Flower lengths were measured in two pure lines, and their F1 and F2 and backcross progenies. To eliminate multiplicative effects, logarithms of the measurements were used. The phenotypic variances were P1 = 48, P2 = 32, F1 = 46, F2 = 130:5, B1 (F1 × P1) = 85:5, and B2(F1 × P2) = 98:5.     (a)  Estimate the environmental variance (VE), the additive genetic variance (VA), and the dominance genetic variance (VD).     (b)  Calculate the degree of dominance.     (c)  Estimate the narrow sense heritability of flower length in the F2.
  6. Let r1, = phenotypic correlation of full sibs, r2 = phenotypic correlation of half sibs, r3 = correlation of offspring with one parent, r4 = correlation of monozygotic twins, and r5 = correlation of dizygotic twins. In the following formulas, determine the values of x and/or y:
      (a)  h2 = x(r1r2)     (b)  h2 = xr1yr3     (c) h2 = x(r4r5)
  7. In the following table, Y represents the average number of bristles on a specific thoracic segment of Drosophila melanogaster in four female offspring and X represents the number of bristles in the mother (dam) of each set of four daughters.
  8. Supplementary Problems

    (a)  Calculate the daughter-dam regression.     (b)   Estimate the heritability of bristle number in this population assuming sX = sY.

  9. A flock of chickens has an average mature body weight of 6.6 lb. Individuals saved for breeding purposes have a mean of 7.2 lb. The offspring generation has a mean of 6.81 lb. Estimate the heritability of mature body weight in this flock.
  10. Yearly wool records (in pounds) are taken from a sample of 10 sheep: 11.8, 8.4, 9.5, 10.0, 10.9, 7.8, 10.8, 8.5, 11.8, 10.5.     (a)  Calculate the range within which approximately 95% of the sheep in this population are expected to be found.     (b)  If the additive genetic variance is 0.60, what is the heritability estimate of wool production in this breed?
  11. Determine     (a)  the dominance variance and     (b)  the environmental variance from the following information: heritability [formula (8:9)] = 0:3, phenotypic variance = 200 lb2, total genetic variance = 100 lb2, and epistatic variance is absent.
  12. Thickness of backfat in a certain breed of swine has been estimated to have a heritability of 80%. Suppose the average backfat thickness of this breed is 1.2 in and the average of individuals selected from this population to be the parents of the next generation is 0.8 in. What is the expected average of the next generation?
  13. The average yearly milk production of a herd of cows is 18,000 lb. The average milk production of the individuals selected to be parents of the next generation is 20,000 lb. The average milk production of the offspring generation is 18,440 lb.     (a)  Estimate the heritability of milk production in this population.     (b)  If the phenotypic variance of this population is 4,000,000 lb2, estimate the additive genetic variance.     (c) Between what two values is the central 68% of the original (18,000 lb average) population expected to be found?
  14. About 1903, Johannsen, a Danish botanist, measured the weight of seeds in the Princess variety of bean. Beans are self-fertilizing and therefore this variety is a pure line. The weights in centigrams (cg) of a small but representative sample of beans are listed below.
  15. Supplementary Problems

    (a)  Calculate the mean and standard deviation for bean weight in this sample.     (b)  Calculate the environmental variance.     (c)  Estimate the heritability of bean weight in this variety.     (d)  If the average bean weight of individuals selected to be parents from this population is 30 cg, predict the average bean weight of the next generation.

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