Question 1

In a population of tomato plants, mean fruit weight is 60 g and h 2 is 0.3. Predict the mean weight of the progeny if tomato plants whose fruit averaged 80 g were selected from the original population and interbred.

Accepted Answer

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Question 2

In a population of 100 inbred, genotypically identical rice plants, variance for grain yield is 4.67. What is the heritability for yield? Would you advise a rice breeder to improve yield in this strain of rice plants by selection?

Accepted Answer

The answer of In a population of 100 inbred, genotypically...

Question 3

Many traits of economic or medical significance are determined by quantitative trait loci (QTLs) in which many genes, usually scattered throughout the genome, contribute to expression.
(a) What general procedures are used to identify such loci?
(b) What is meant by the term cosegregate in the context of QTL mapping? Why are markers such as RFLPs, SNPs, and microsatellites often used in QTL mapping?

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Question 4

A 3-inch plant was crossed with a 15-inch plant, and all F 1 plants were 9 inches. The F 2 plants exhibited a "normal distribution," with heights of 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 inches.
(a) What ratio will constitute the "normal distribution" in the F 2 ?
(b) What will be the outcome if the F1 plants are testcrossed with plants that are homozygous for all nonadditive alleles?

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Question 5

In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F 1 are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F 2 individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

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Question 6

Type A1B brachydactyly (short middle phalanges) is a genetically determined trait that maps to the short arm of chromosome 5 in humans. If you classify individuals as either having or not having brachydactyly, the trait appears to follow a single-locus, incompletely dominant pattern of inheritance. However, if one examines the fingers and toes of affected individuals, one sees a range of expression from extremely short to only slightly short. What might cause such variation in the expression of brachydactyly?

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Question 7

In a series of crosses between two true-breeding strains of peaches, the F 1 generation was uniform, producing 30-g peaches. The F 2 fruit mass ranges from 38 to 22 g at intervals of 2 g.
(a) Using these data, determine the number of polygenic loci involved in the inheritance of peach mass.
(b) Using gene symbols of your choice, give the genotypes of the parents and the F 1 
.

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The answer of In a series of crosses between two...

Question 8

Students in a genetics laboratory began an experiment in an attempt to increase heat tolerance in two strains of Drosophila melanogaster. One strain was trapped from the wild six weeks before the experiment was to begin; the other was obtained from a Drosophila repository at a university laboratory. In which strain would you expect to see the most rapid and extensive response to heat-tolerance selection, and why?

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Question 9

Consider a true-breeding plant, AABBCC , crossed with another true-breeding plant, aabbcc , whose resulting offspring are AaBbCc. If you cross the F 1 generation, and independent assortment is operational, the expected fraction of offspring in each phenotypic class is given by the expression N!/M!(N - M)! where N is the total number of alleles (six in this example) and M is the number of uppercase alleles. In a cross of AaBbCc × AaBbCc , what proportion of the offspring would be expected to contain two uppercase alleles?

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Question 10

Canine hip dysplasia is a quantitative trait that continues to affect most large breeds of dogs in spite of approximately 40 years of effort to reduce the impact of this condition. Breeders and veterinarians rely on radiographic and universal registries to facilitate the development of breeding schemes for reducing its incidence. Data (Wood and Lakhani. 2003. Vet. Rec. 152: 69-72) indicate that there is a "month-of-birth" effect on hip dysplasia in Labrador retrievers and Gordon setters, whereby the frequency and extent of expression of this disorder vary depending on the time of year dogs are born. Speculate on how breeders attempt to "select" out this disorder and what the month-of-birth phenomenon indicates about the expression of polygenic traits.

Accepted Answer

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Question 11

Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.
  
  (a) Considering that differences in control values represent year-to-year differences in greenhouse conditions, calculate (in mm) the average response to selection over the three-year period.
(b) Calculate the realized heritability for each year and the overall realized heritability.
(c) Assuming that the realized heritability in phlox is relatively high, what factors might account for such a high response?
(d) In terms of evolutionary potential, is a population with high heritability likely to be favored compared to one with a low realized heritability?

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Question 12

In 1988, Horst Wilkens investigated blind cavefish, comparing them with members of a sibling species with normal vision that are found in a lake [Wilkens, H. (1988). Ecol. Biol. 23: 271-367]. We will call them cavefish and lakefish. Wilkens found that cavefish eyes are about seven times smaller than lakefish eyes. F 1 hybrids have eyes of intermediate size. These data, as well as the F 1 × F 1 cross and those from backcrosses (F 1 × cavefish and F 1 × lakefish), are depicted below. Examine Wilkens's results and respond to the following questions:
(a) Based strictly on the F 1 and F 2 results of Wilkens's initial crosses, what possible explanation concerning the inheritance of eye size seems most feasible?
(b) Based on the results of the F 1 backcross with cavefish, is your explanation supported? Explain.
(c) Based on the results of the F 1 backcross with lakefish, is your explanation supported? Explain.
(d) Wilkens examined about 1000 F 2 progeny and estimated that 6-7 genes are involved in determining eye size. Is the sample size adequate to justify this conclusion? Propose an experimental protocol to test the hypothesis.
(e) A comparison of the embryonic eye in cavefish and lake-fish revealed that both reach approximately 4 mm in diameter. However, lakefish eyes continue to grow, while cavefish eye size is greatly reduced. Speculate on the role of the genes involved in this problem.

Accepted Answer

The answer of In 1988, Horst Wilkens investigated blind cavefish,...

Quiz 23: Quantitative Genetics and Multifactorial

In a population of tomato plants, mean fruit weight is 60 g and h 2 is 0.3. Predict the mean weight of the progeny if tomato plants whose fruit averaged 80 g were selected from the original population and interbred.

In a population of 100 inbred, genotypically identical rice plants, variance for grain yield is 4.67. What is the heritability for yield? Would you advise a rice breeder to improve yield in this strain of rice plants by selection?