without the benefit of knowing what was occurring within the nucleus or that chromosomes
existed. Gregor Mendel’s work in genetics was relatively obscure in his own day but was
“rediscovered” in the twentieth century (see Bateson 1909, Sutton 1903).
Mendel’s choice of working with peas was a good one, since the pea plants he used
differed from one another in several relatively simple phenotypic traits. Seed shape and
color, pod shape and color, plant height, and flower position were the traits that he
traced over generations of sexual reproduction (Mendel 1866). The pea plants had different
variants for a given trait (Fig. 2.4). For example, some of the pea plants had yellow seeds,
while others had green seeds. Each trait that Mendel followed was controlled by a single
gene, and the traits themselves were often discrete. That is, seeds could be scored as
either yellow or green, and not a mixed or splotched variant that was in between the original
parents.Mendelian traitsare controlled by a single gene, and therefore the protein product
from a single gene directly leads to the characteristic phenotype. This is one of the most
important concepts in plant biotechnology since all transgenic plants produced to date
have traits controlled by single transgenes. Mendelian traits may have multiple different
versions that make different proteins with varying characteristics, but the gene that controls
the trait is at a single location within a chromosome in the genome called alocus(Fig. 2.2c).
The different versions of each gene are calledalleles, and differ from one another in the
sequence of DNA at that chromosomal locus. Mendelian traits are also characterized by
discrete variation, where the different phenotypes of the trait can be broken into obvious
categories. In the example of pea plant height, tall versus short plant type is determined
by the genotype at a single genetic locus that controls height.
As you will see throughout this book, most traits are more complex than Mendelian traits
because they are controlled by the gene products of many genes, and hence are calledpoly-
genic traits. Polygenic traits exhibitcontinuous variation, where the trait can show a wide
range of phenotypes.Multifactorial traitsare controlled by multiple genes and the environ-
ment in which the plant is grown. Multifactorial traits also exhibit continuous variation, and
will vary with the environmental conditions. Polygenic and multifactorial traits will be dis-
cussed specifically in Chapter 3 of this book. The traits that Mendel followed had two
specific characteristics; they had discrete variation and were controlled by the action of a
single gene.
Mendel was very observant, and was a good botanist. His choice of peas was fortuitous
in that peas normally self-fertilize, which made all of his interpretations of transmission
Figure 2.4.Traits of the pea plant used by Mendel to discover the genetic laws of segregation and
independent assortment. Each trait had two phenotypes: one controlled by a dominant allele and
one by a recessive allele.
26 MENDELIAN GENETICS AND PLANT REPRODUCTION