Principles of Plant Genetics and Breeding

Types of variation among plants
As previously indicated, the phenotype (the observed
trait) is the product of the genotype and the environment
(P = G + E). The phenotype may be altered by
altering G, E, or both. There are two fundamental
sources of change in phenotype – genotype and the
environment – and hence two kinds of variation,
genetic and environmental. Later in the book, an
additional source of change, G × E (interaction of the
genotype and the environment) will be introduced.
Environmental variation
When individuals from a clonal population (i.e., identical
genotype) are grown in the field, the plants will
exhibit differences in the expression of some traits
because of non-uniform environments. The field is
often heterogeneous with respect to plant growth factors
– nutrients, moisture, light, and temperature. Some
fields are more heterogeneous than others. Sometimes,
non-growth factors may occur in the environment and
impose different intensities of environmental stress on
plants. For example, disease and pest agents may not
uniformly infect plants in the field. Similarly, plants that
occur in more favorable parts of the field or are impacted
to a lesser degree by an adverse environmental factor
would perform better than disadvantaged plants. That
Parent
F 1
F 2
(a)
VARIATION 79
is, even clones may perform differently under different
environments, and inferior genotypes can outperform
superior genotypes under uneven environmental conditions.
If a breeder selects an inferior genotype by
mistake, the progress of the breeding program will be
slowed. Consequently, plant breeders use statistical
tools and other selection aids to help in reducing the
chance of advancing inferior genotypes, and thereby
increase progress in the breeding program.
As previously noted, environmental variation is not
heritable. However, it can impact on heritable variation
(see below). Plant breeders want to be able to select a
plant on the basis of its nature (genetics) not nurture
(growth environment). To this end, evaluations of
breeding material are conducted in as uniform an
environment as possible. Furthermore, the selection
environment is often similar to the one in which the
crop is commercially produced.
Genetic variability
Variability that can be attributed to genes that encode
specific traits, and can be transmitted from one generation
to the next, is described as genetic or heritable
variation. Because genes are expressed in an environment,
the degree of expression of a heritable trait is
impacted by its environment, some more so than others
(Figure 5.2). Heritable variability is indispensable to
Figure 5.2 Environmental effect on gene expression: phenotype = genotype + environment. Some traits are influenced a
lot more than others by the environment. Cross (a) has small environmental influence such that the phenotypes are
distinguishable in the F 2 ; in cross (b) the environmental influence is strong, resulting in more blurring of the differences
among phenotypes in the segregating population.
Parent
F 1
F 2
(b)