color. These other differences may or may not be noticeable, and they may or may not be
present within a specific set of germplasm. Thus, in some populations, seed color could be a
polygenic trait. Indeed, a pea breeder might make a cross between a yellow-seeded variety
and a green-seeded variety, then try to select progeny that had even greener seeds than those
of the original green parent. Why? Because the yellow-seeded variety might contain alleles
at loci other than the primary seed color locus that are capable of enhancing the green color
in the presence of the green alleles at the primary locus. Thus, seed color can be either quali-
tative and monogenic (simple inheritance), or quantitative and polygenic (complex inheri-
tance) or both, depending on circumstance and on the germplasm being investigated.
3.2.2 Phenotype versus Genotype
An important term used throughout this book isphenotype, which simply means “what
something looks like.” We often speak about the phenotype of a specific trait, in which
case it takes on units of measurement. For example, the phenotype of a quantitative trait
such as seed weight in wheat might range between 30 and 80 mg. The termphenotypeis
also used to distinguish what a plant looks like from its genotype(what genes are
present) orgenotypic value(what we would expect the phenotype to be if we could
predict it exactly from the genotype). A fundamental concept in plant breeding is that gen-
otypic value is something that we try to measure and predict. If we could identify or control
all the unpredictable effects of error and environment, then the phenotype of a plant (P)
would be equal to its genotype (G) plus the effects caused by error and environment
(E). Virtually all of the fancy equations that you will see in plant breeding books are deri-
vations of this basic formula
P¼GþEor more precisely
P¼SGþEwhereSindicates that genetic effects may be summed over multiple genes, as they are
in Figure 3.1.
The equations above refer to the genotypic or phenotypic values of a single plant or
observation. However, breeders work with populations of many plants, and they often sum-
marize a set of observations by calculating thevariance, which is simply a mathematical
formalization of variability, and genetic variability is the key to creating varieties
through artificial selection. The basic breeding equation can also be written to describe a
population of plants in terms of phenotypic variance (VP), genetic variance (VG) and
environmental variance (VE), such that
VP¼VGþVEIt is imperative for any breeder to understand the relative proportion of genetic variance
that contributes to phenotypic variance for a given trait. This concept is formalized using
the termheritability(H), which, in its simplest form, is measured as
H¼VG
VP3.2. CENTRAL CONCEPTS IN PLANT BREEDING 51