parental stock. Other usage of the term occurs in the literature.
3.11.1 Methods of population improvement
Some form of evaluation precedes selection. A breeding material is selected after evaluating the variability available. Similarly, advancing plants from one generation to the next is preceded by an evaluation to determine individuals to select. In self‐pollinated species, individuals are homozygous and when used in a cross their genotype is precisely reproduced in their progeny. A progeny test is hence adequate for evaluating an individual's performance. However, open‐pollinated species are heterozygous plants and are further pollinated by other heterozygous plants growing with them in the field. Progeny testing is hence not adequately evaluative of the performance of individual plants of such species. A more accurate evaluation of performance may be achieved by using pollen (preferably from a homozygous source – inbred line) to pollinate the plants. As previously described, the method of evaluating the performance of different mother plants in a comparative way using a common pollen source (tester line) is called a test cross. The objective of such a test is to evaluate the performance of a parent in a cross, a concept called combining ability.
The methods used by plant breeders in population improvement may be categorized into two groups, based on the process for evaluating performance. One group of methods is based solely on phenotypic selection and the other on progeny testing (genotypic selection). The specific methods include mass selection, half‐sib, full‐sib, recurrent selection, and synthetics.
Key references and suggested reading
1 Anderson, J.B. and Kohn, L.M. (1998). Genotyping, gene genealogies and genomics bring fungal population genetics above ground. Trends in Ecology and Evolution 13 (11): 444–449.
2 Ayala, F.J. and Campbell, C.A. (1974). Frequency‐dependent selection. Annual Review of Ecology, Evolution, and Systematics 5: 115–138.
3 Brown, J.K.M. (1996). The choice of molecular marker methods for population genetic studies of plant pathogens. New Phytologist 133: 183–195.
4 Cornelius, P.L. and Dudley, J.W. (1974). Effects of inbreeding by selfing and full‐sib mating in a maize population. Crop Science 14: 815–819.
5 Crow, J.F. and Kimura, M. (1970). An Introduction to Population Genetics Theory. New York: Harper and Row.
6 Falconer, D.S. (1981). Introduction to Quantitative Genetics, 2e. NY: Longman.
7 Hartl, D.L. and Clark, A.G. (1997). Principles of Population Genetics, 3e. Sunderland, MA: Sinauer Associates Inc.
8 Hayward, M.D. and Breese, E.L. (1993). Population structure and variability. In: Plant Breeding: Principles and Practices (eds. M.D. Hayward, N.O. Bosemark and I. Ramagosa), 16–29. London: Chapman and Hall.
9 Hedrick, P.W. (1985). Genetics of Populations. Boston: Jones & Bartlett.
10 Li, C.C. (1976). A First Course in Population Genetics. Pacific Grove, CA: Boxwood.
11 McDonald, B.A. and Linde, C. (2002). Pathogen population genetics, evolutionary potential, and durable resistance. Annual Review of Phytopathology 40: 349–379.
12 McDonald, B.A. and McDermott, J.M. (1993). Population genetics of plant pathogenic fungi. Bioscience 43: 311–319.
13 Milgroom, M.G. and Fry, W.E. (1997). Contributions of population genetics to plant disease epidemiology and management. Advances in Botanical Research 24: 1–30.
Internet resources
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hardy_Weinberg.html – Excellent discussion of population genetics.
Outcomes assessment
Part A
Please answer these questions true or false:
1 Inbreeding promotes heterozygosity.
2 Naturally cross‐breeding species are more susceptible to inbreeding than naturally self‐pollinated species.
3 In Hardy‐Weinberg equilibrium gene frequencies add up to unity.
4 Open‐pollinat ed species can be improved by mass selection.
Part B
Please answer the following questions:
1 Define the terms (a) population and (b) gene pool.
2 Give three major factors that influence the genetic structure of a population during the processes of transmission of genes from one generation to another.
3 Explain the phenomenon of inbreeding depression.
4 Distinguish between assortative and disassortative matings.
5 Discuss the main types of mating systems used by plant breeders to effect inbreeding.
Part C
Please write a brief essay on each of the following topics:
1 Discuss the Hardy‐Weinberg equilibrium and its importance in breeding cross‐pollinated species.
2 Discuss the consequences of inbreeding.
3 Discuss the concept of combining ability.
4 Discuss the application of inbreeding in plant breeding.
Purpose and expected outcomes
Most of the traits that plant breeders are interested in are quantitatively inherited. It is important to understand the genetics that underlie the behavior of these traits in order to develop effective approaches for manipulating them. After studying this chapter, the student should be able to:
1 Define quantitative genetics and distinguish it from population genetics.
2 Distinguish between qualitative traits and quantitative traits.
3 Discuss polygenic inheritance.
4 Discuss gene action.
5 Discuss the variance components of quantitative traits.
6 Discuss the concept of heritability of traits.
7 Discuss selection and define the “breeders’ equation.”
8 Discuss the concept of general worth of a plant.
9 Discuss combining ability.
4.1 What is quantitative genetics?
