3.2.4 Other Topics in Population and Quantitative Genetics
Refer back to Figure 3.1, and try to imagine factors that might complicate the situation that
is illustrated. Rather than two genes affecting this trait, there could be dozens of genes.
Different genes could have effects of different magnitudes, and there could also be an
environmental influence. Rather than beingunlinked(assorting independently), some of
the loci might belinkedtogether on the same chromosomes. This would result in some
combinations of parental alleles being more frequent than others. The effects of individual
alleles may not be completelyadditiveas they are in this figure (i.e., the genotypic value is
found by adding up the individual genotypic effects at each locus). If the genotypic value
of the heterozygote is not equal to the average of the homozygotes, we refer to this as
dominance(meaning that one allele has a dominant effect over another). If alleles at differ-
ent loci interact (i.e., if the total genotypic value is different from the sum of the genotype
values of the individual loci), we call thisepistasis. Many of these factors will tend to
produce a histogram of phenotypes that is more continuous (smoother) than the distri-
bution shown in Figure 3.1, but they can also cause the shape of the distribution to
deviate from the normal (bell-shaped) distribution that results when all genes have
uniform, additive effects. All of these concepts are simplest to study in a diploid plant.
However, many crop plants such as potato and strawberry are not diploids, but ratherpoly-
ploids. Inheritance in polyploids is considerably more complex than in diploids because
there can be more than two different alleles at a given locus, and they can interact in
many different ways.
There is an entire field calledquantitative geneticsthat is dedicated to the study and pre-
diction of genetic effects that underlie quantitative traits, and any serious study of plant
breeding must be accompanied by further study of quantitative genetics [e.g., see the text
by Wrike and Weber (1986)]. An excellent introduction to many modern concepts in quan-
titative genetics is provided by Barton and Keightley (2002). Quantitative genetics builds
on the topic ofpopulation genetics(the study of gene flow in populations), and many
curriculaseparate these topics into different courses of study.
The study of quantitative genetics has been given a significant boost since the mid-1980s
by the discovery of molecular markers, and the ability to produce high-density molecular
maps of where these markers and genes lie within plant chromosomes. When mapped
molecular markers are segregating in the same population as a quantitative trait, it is often
possible to find discrete relationships between map locations and individual genes that
control the quantitative trait. This procedure, known asquantitative trait locus(QTL)analy-
sis, is the key to understanding the genetic control of many complex traits. It is also the
concept that lies at the heart of marker-assisted breeding (the use of molecular markers to
assist in the selection of linked traits). A detailed discussion of QTL analysis is provided
by Paterson (1998), but an Internet search of “QTLþyour favorite plant species” may
direct you to primary literature regarding the discovery of QTL in your species of choice.
3.2.5. The Value of a Plant Variety Depends on Many Traits
If a melon breeder had nothing to worry about besides fruit size, then melon breeders might
have finished their jobs long ago, and/or melons might now be approaching the size of
small cars. However, plant varieties are often bought and sold in an open market, and
the value of a plant variety is subject to complex and changing industry and consumer pre-
ferences. Some of these preferences are mentioned in Section 3.3. There can be no perfect
3.2. CENTRAL CONCEPTS IN PLANT BREEDING 55