George Acquaah - Principles of Plant Genetics and Breeding

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 testis 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.

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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hardy_Weinberg.html– Excellent discussion of population genetics.

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.

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.

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.

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.

Population genetics and quantitative genetics are closely related fields, both dealing with the genetic basis of phenotypic variation among the individuals in a population. Population genetics traditionally focuses on frequencies of alleles and genotypes, whereas quantitative genetics focuses on linking phenotypic variation of complex traits to its underlying genetic basis to enable researchers to better understand and predict genetic architecture and long‐term change in populations (to predict the response to selection given data on the phenotype and relationships of individuals in the population). Historically, quantitative genetics has its roots in statistical abstractions of genetic effects, first described by Karl Pearson and Ronald Fisher in the early 1900s. The foregoing represents the classical view of quantitative genetics.

The modern molecular view of quantitative genetics focuses on the use of molecular genetics tools (genomics, bioinformatics, computational biology, etc.) to reveal links between genes and complex phenotypes (quantitative traits). Genes that control quantitative traits are called quantitative trait loci (QTL). Molecular‐based QTL analyses are being used to evaluate the coupling associations of the polymorphic deoxyribonucleic acid (DNA) sites with phenotypic variations of quantitative and complex traits and analyze their genetic architecture. There is evidence of a paradigm shift in the field of quantitative genetics. In this chapter, both classical ( Sections 4.2– 4.2.19) and molecular ( Sections 4.3– 4.7) quantitative genetics are discussed. Discussions in this section will include genetic and environmental variances, relationships and genetic diversity, linkage, and epistatic issues in populations.