Ch. 25 - Quantitative Genetics and Multifactorial Traits

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook

Klug 12th Edition

Ch. 25 - Quantitative Genetics and Multifactorial Traits

Problem 27

Chapter 25, Problem 27

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 a modified form and content.
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

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Identify the control group values for flower size (diameter) for each of the three years to understand the baseline measurements under normal greenhouse conditions.

Determine the selected group values for flower size for each year, which represent the plants subjected to artificial selection for increased or decreased flower size.

Calculate the response to selection for each year by subtracting the control group mean from the selected group mean:

R = \(\bar{X}\)_{selected} - \(\bar{X}\)_{control}

. This gives the change in flower size due to selection in that year.

Sum the yearly responses to selection and then divide by the number of years (three) to find the average response to selection over the three-year period:

= \(\frac{\sum R_i}{3}\)

Interpret the average response to selection as the mean change in flower diameter (in mm) attributable to artificial selection across the three years, accounting for environmental variation reflected in the control values.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Realized Heritability

Realized heritability measures the proportion of phenotypic variation in a trait that is due to additive genetic factors, estimated from the response to selection relative to the selection differential. It is calculated by dividing the response to selection by the selection differential, providing insight into how much a trait can evolve under selection.

Response to Selection

Response to selection is the change in the average phenotype of a population after one generation of selection. It reflects how much the trait mean shifts due to selective breeding and is influenced by genetic variation and selection intensity. Calculating the average response over multiple years helps understand the trait's evolutionary potential.

Environmental Variation and Control Values

Environmental variation affects phenotypic traits independently of genetics, causing year-to-year differences in control group measurements. Adjusting for these differences is essential to isolate the genetic response to selection, ensuring that observed changes in traits like flower size are due to genetic factors rather than environmental fluctuations.

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Textbook Question

Consider a true-breeding plant, AABBCC, crossed with another true-breeding plant, aabbcc, whose resulting offspring are AaBbCc. If you cross the F₁ 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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Textbook Question

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.

Calculate the realized heritability for each year and the overall realized heritability.

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Textbook Question

In a series of crosses between two true-breeding strains of peaches, the F₁ generation was uniform, producing 30-g peaches. The F₂ fruit mass ranges from 38 to 22 g at intervals of 2 g.

Using gene symbols of your choice, give the genotypes of the parents and the F₂.

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Textbook Question

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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Textbook Question

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.

Assuming that the realized heritability in phlox is relatively high, what factors might account for such a high response?

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Textbook Question

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.

432

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