Principles of Plant Genetics and 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 the concept of combining ability.
What is quantitative genetics?
Genetics has several branches, including population
genetics, quantitative genetics, biometric genetics, and
molecular genetics. Population genetics is an extension
of Mendelian genetics applied at the population level.
Population genetics does not assign a genotypic or
numerical value to each of the individuals (genotypes) in
the population (except in the case of coefficients of
selection). Quantitative genetics, on the other hand, is
a branch of genetics in which individual genotypes are
unidentified, and the traits of individuals are measured.
Genotypic values are assigned to genotypes in the population.
Quantitative genetics emphasizes the role of
selection in controlled populations of known ancestry.
Some topics of population genetics are often discussed
in quantitative genetics books, partly because population
genetics is basic to quantitative genetics.
8
Introduction to
quantitative genetics
A quantitative geneticist observes the phenotype, a product
of the genotype and the environment. The genotypic
array depends on mating systems and genetic linkage
relationships, as well as on allelic frequencies, which in
turn are impacted by mutation, migration, random drift,
and selection (see Chapter 7). To make effective observations
about phenotypes, the quantitative geneticist
has to make assumptions about the mating system,
allelic frequency altering forces, and the environment.
Common assumptions of quantitative genetic analysis
are as follow:
1 Reference population defined. Allelic and genotypic
frequencies can only be defined with respect to a
specified population. The researcher should define a
base reference population. All inferences made about
the estimates should depend upon the composition
of this reference population.