Quantitative Genetics Practice Test (page 3)
Review the following concepts if needed:
- Qualitative vs. Quantitative Traits for Genetics
- Polygenic Traits for Genetics
- The Normal Distribution for Genetics
- Types of Gene Action for Genetics
- Heritability for Genetics
- Selection Methods for Genetics
- Mating Methods for Genetics
Quantitative Genetics Practice Test
For each of the following definitions, give the appropriate term and spell it correctly. Terms are single words unless indicated otherwise.
- The kind of phenotypic variation associated with quantitative traits. (One or two words.)
- A bell-shaped distribution of continuous phenotypic variation. (One or two words.)
- A squared standard deviation.
- A type of allelic interaction in which the phenotype of a heterozygote is outside the phenotypic limits of the corresponding homozygotes.
- The proportion of the phenotypic variance of a trait that is attributable to gene effects.
- A statistic expressing how much (on average) one sample variable may be expected to change per unit change in some other variable. (Two words.)
- A statistical measurement of how closely two sets of sample data are associated, having limits ±1. (Two words.)
- A method of estimating the breeding value of an individual by the performance or phenotype of its offspring. (Two words.)
- The mating of individuals that are more closely related than the average of the population to which they belong.
- The superior phenotypic quality of heterozygotes relative to that of homozygotes, commonly called "hybrid vigor."
Choose the one best answer.
For problems 1–3, use the following information. Two pure lines of corn have mean cob lengths of 9 and 3 in, respectively. The polygenes involved in this trait all exhibit additive gene action.
- Crossing these two lines is expected to produce a progeny with mean cob length (in inches) of (a) 12.0 (b) 7.5 (c) 6.0 (d) 2.75 (e) none of the above
- If the variation in F1 cob length ranges from 5.5 to 6.5 in, this variation is estimated to be due to segregation at (a) two loci (b) three loci (c) four loci (d) five loci (e) none of the above
- If only two segregating loci contribute to cob length, and we represent the parental cross as AABB (9-in average cob length) × aabb(3-in average), the fraction of the F2 expected to be 4.5 in is (a) 1/8 (b) 1/16 (c) 3/32 (d) 3/16 (e) none of the above
- If a mouse population has an average adult body weight of 25 g with a standard deviation of ±3 g, the percentage of the population expected to weigh less than 22 g is approximately (a) 16 (b) 33 (c) 68 (d) 50 (e) 25
- With reference to problem 4 above, if the genetic variance in mouse body weight is 2.7, the environmental variance is approximately (a) 22.3 (b) 6.3 (c) 0.3 (d) 3.3 (e) none of the above
- In another population of mice, the total genetic variance of adult bodyweight is 4g2 and the environmental variance is 12g2. The broad estimate of heritability for this trait in this population is approximately (a) 0.15 (b) 0.20 (c) 0.25 (d) 0.33 (e) none of the above
- If the correlation coefficient between body weight of full sibs is 0.15, then the heritability of this trait in this population is (a) less than 0.15 (b) 0.25 (c) 0.3 (d) 0.6 (e) none of the above
- The selection differential (in grams) is (a) 0.5 (b) 4 (c) 3.5 (d) 2 (e) none of the above
- The genetic gain (in grams) is (a) 0.5 (b) 4 (c) 3.5 (d) 2 (e) none of the above
- The heritability estimate for adult body weight in this population is (a) 0.050 (b) 0.625 (c) 0.125 (d) 0.250 (e) none of the above
For problems 8-10, use the following information. A population of adult mice has a mean body weight of 30 g. The average weight of mice selected for breeding purposes is 34g. The progeny produced by random mating among the selected parents average 30.5g.
Polygenic Traits Questions
- 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?
- 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
- 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.
- 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).
- 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
- 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.
- Let Vi = phenotypic variance between identical twins, Vf = phenotypic variance between fraternal twins, and heritability (h2) = (Vf – Vi)/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.
- 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.
- 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.
- 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(r1 – r2) (b) h2 = xr1 – yr3 (c) h2 = x(r4 – r5)
- 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.
- 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.
- 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?
- 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.
- 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?
- 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?
- 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.
(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.
(a) Calculate the daughter-dam regression. (b) Estimate the heritability of bristle number in this population assuming sX = sY.
(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.
Selection Methods Questions
- The length of an individual beetle is 10.3mm or 0.5 when expressed in "standardized" form. The average measurement for this trait in the beetle population is 10.0 mm. What is the variance of this trait?
- Given the swine selection index I = 0.14W – 0.27S, where W is the pig's own 180-day weight and S is its market score. (a) Rank the following three animals according to index merit:
- A beef cattle index (I) for selecting replacement heifers takes the form I = 6 + 2WW' + WG', where WW' is weaning weight in standardized form and WG' is weaning grade in standardized form. The average weaning weight of the herd = 505 lb with a standard deviation of ±34:5 lb. The average weaning grade (a numerical score) is 88.6 with a standard deviation of ±2:1. Which of the following animals has the best overall merit?
- Suppose 360 ewes (female sheep) are available for proving sires. All ewes lamb; 50% of ewes lambing have twins. The 10 rams with the highest progeny test scores will be kept as flock sires. How much selection can be practiced among the progeny-tested individuals; i.e., what proportion of those tested can be saved if a test requires (a) 18 progeny, (b) 12 progeny, (c) 6 progeny?
- During the same year, three dairy bulls were each mated to a random group of cows. The number of pounds of butterfat produced by the dams and their daughters (corrected to a 305-day lactation at maturity with twice daily milking) was recorded as shown below.
(b) If differences in index score are 20% heritable, and parents score 3.55 points higher than the average of the population, how much increase in the average score of the progeny is expected?
(a) Calculate the sire index for each of the three sires. (b) Which sire would you save for extensive use in your herd?
Mating Methods Questions
- A is linebred to B in the following pedigree. Calculate the inbreeding coefficient of A.
- Calculate the inbreeding of A in the following. (Hint: There are nine pathways between B and C)
- The yield of seed (in bushels per acre) and plant height (in centimeters) was measured on several generations of corn. Calculate by formula (8.27), (a) the amount of heterosis in the F1 resulting from crossing the parental varieties with the inbreds, (b) the yield and height expectations of the F2.
- continuous variation
- normal (or Gaussian) distribution
- heritability (broad definition)
- regression coefficient
- correlation coefficient
- progeny test
- e (environmental)
- e (1/4)
- Eight pairs of factors
- Three or four loci (pairs of alleles)
The Normal Distribution
- (a) 216 lb (b) ±6.58 lb (c) 229.16lb
- ±1000 lb/acre
- 4.16 or approximately four gene pairs
Type of Gene Action
- (a) Both parents, F1 = 36 units (b) Both parents = 36 units, F1 = 48 units
- (a) Heritability estimates measure the proportion of the total phenotypic variation that is due to genetic variation for a trait among individuals of a population. There is no genetic variation in a pure line (heritability = 0), but blood groups (for example) would still be 100% determined by genes. (b) Suppose that a group of identical twins were divided, one member of each pairto the two populations A and B. Each population would then have the same genetic constitution. If population A is not given equal social, educational, and vocational opportunities with population B, then A might be expected to show lower average IQ. In other words, the average IQs of these populations would be reflective solely of nongenetic (environmental) differences regardless of the heritability estimates made in each population. (c) The answer to part (b) demonstrates that the difference between phenotypic averages of two populations does not necessarily imply that one population is genetically superior to the other. Important environmental differences may be largely responsible for such deviations. We could imagine that a pure line (heritability = 0) would be very well suited to a particular environment, whereas a highly genetically heterogeneous population with a high heritability for the same trait might be relatively poorly adapted to that same environment. In other words, the high heritabilities for IQ within populations A and B reveal nothing about the causes of the average phenotypic differences between them.
- (a) VE = 42.0, VA = 77.0, VD = 11.5 (b) 0.55 (c) 0.59
- (a) x = 4 (b) x = 4, y = 2 (c) x = 2
- (a) b = 0.22 (b) h2 = 2b = 0.49
- (a) 7.17–12.83 lb (b) 0.3
- (a) 40 lb2 (b) 100 lb2
- 0.88 in
- (a) 0.22 (b) 880, 000 lb2 (c) 16, 000–20, 000 lb
- (a) = 25 cg, s = ±3.94 cg (b) 15.53 cg2, note that this is the square of the phenotypic standard deviation in part (a). In pure lines, all of the variance is environmentally induced. (c) h2 = 0, since a pure line is homozygous; there is no genetic variability. (d) = 25 cg; no genetic gain can be made by selecting in the absence of genetic variability.
- 0.36 mm2
- (a) Y = 23.34, Z = 19.90, X = 17.84 (b) 0.71 point
- IA = 6.526, IB = 6.041, A excels in overall merit.
- (a) 1/3 (b) 22.2% (c) 1/9
- (a) A = 630.0, B = 615.0, C = 597.5 (b) Sire A
- (a) Heterosis for seed yield = 22.2 bushels/acre, for plant height = 28 cm (b) 60.3 bushels/acre, 243 cm
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