Principles of Plant Genetics and Breeding

The Ac element has an open reading frame. The activities
of corn transposable elements are developmentally
regulated. That is, the transposable elements transpose
and promote genetic rearrangements at only certain
specific times and frequencies during plant development.
Transposition involving the Ac–Ds system is
observed in corn as spots of colored aleurone. A gene
required for the synthesis of anthocyanin pigment is
inactivated in some cells whereas other cells have normal
genes, resulting in spots of pigment in the kernel
(genetic mosaicism).
Biotechnology for creating genetic
variability
Gene transfer
The rDNA technology is state-of-the-art in gene transfer
to generate genetic variability for plant breeding.
With minor exceptions, DNA is universal. Consequently,
DNA from an animal may be transferred
to a plant! The tools of biotechnology may be used to
incorporate genes from distant sources into adapted
cultivars. An increasing acreage of cotton, soybean, and
maize are being sown to genetically modified (GM) cultivars,
indicating the importance of this technology for
creating variability for plant breeding. Economic gene
transfers have been made from bacteria to plants to confer
disease and herbicide resistance to plants. The most
common GM products on the market are Roundup
Ready® cultivars (e.g., cotton, soybean) with herbicide
tolerance, and Bt products (e.g., corn) with resistance
to lepidopteran pests. The technique of site-directed
mutagenesis allows scientists to introduce mutations
into specified genes, primarily for the purpose of studying
gene function, and not for generating variability for
breeding per se. Other tissue-culture-based techniques
include protoplast fusion, cybrid formation, and the use
of transposons. Chapter 14 is devoted to the application
of biotechnology in plant breeding.
Somaclonal variation
In vitro culture of plants is supposed to produce clones
(genetically identical derivatives from the parent material).
However, the tissue culture environment has been
known to cause heritable variation called somaclonal
variation. The causes cited for these changes include
karyotypic changes, cryptic chromosomal rearrangements,
somatic crossing over and sister chromatid
exchange, transposable elements, and gene amplification.
VARIATION 83
Some of these variations have been stable and fertile
enough to be included in breeding programs.
Scale of variability
As previously indicated, biological variation can be
enormous and overwhelming to the user. Consequently,
there is a need to classify it for effective and
efficient use. Some variability can be readily categorized
by counting and arranging into distinct non-overlapping
groups; this is said to be discrete or qualitative variation.
Traits that exhibit this kind of variation are
called qualitative traits. Other kinds of variability occur
on a continuum and cannot be placed into discrete
groups by counting. There are intermediates between
the extreme expressions of such traits. They are best categorized
by measuring or weighing and are described as
exhibiting continuous or quantitative variation. Traits
that exhibit this kind of variation are called quantitative
traits.
However, there are some plant characters that may be
classified either way. Sometimes, for convenience, a
quantitative trait may be classified as though it were
qualitative. For example, an agronomic trait such as
earliness or plant maturity is quantitative in nature.
However, it is possible to categorize cultivars into maturity
classes (e.g., in soybean, maturity classes range from
000 (very early) to VIII (very late)). Plant height can be
treated in a similar fashion, and so can seed coat color
(expressed as shades of a particular color).
Qualitative variation
Qualitative variation is easy to classify, study, and utilize
in breeding. It is simply inherited (controlled by one
or a few genes) and amenable to Mendelian analysis
(Figure 5.6). Examples of qualitative traits include diseases,
seed characteristics, and compositional traits.
Because they are amenable to Mendelian analysis, the
chi-square statistical procedure may be used to determine
the inheritance of qualitative genes. The success of
gene transfer using molecular technology so far has
involved the transfer of single genes (or a few at best),
such as the Bt and Ht (herbicide-tolerant) products.
Breeding qualitative traits
Breeding qualitative traits is relatively straightforward.
They are readily identified and selected, although
breeding recessive traits is a little different from breeding
dominant traits (Figure 5.7). It is important to have