Horticulture Principles and Practices

FIGURE 6–5 A gene gun. This
tabletop model uses compressed
helium gas to propel the DNA.
(Source: George Acquaah)
SUMMARY
208 Chapter 6 Breeding Horticultural Plants
Plant improvement involves the manipulation of the genome of the plant with a specific
goal in mind. A good understanding of genetics helps breeders to be effective and
efficient in their endeavors. DNA, hereditary material, is located mostly in the nuclei of
plants in linear structures called chromosomes. Genes, the factors that condition traits,
are pieces of DNA arranged in a linear order in chromosomes. Gene—not traits—are
inherited by the offspring from their parents. The transmission of genes in this fashion is
governed by certain laws that were discovered by Mendel: gene pairs segregate during
cell division so that only one from each pair ends up in each gamete, and genes assort
independently during meiosis. Some genes (dominant) mask the expression of others
(recessive) when they occur at the same locus.
Meiosis is the major source of biological variation. However, the ultimate source
of variation is mutation. The expression of genes is subject to the environment in which
they occur. Certain traits are governed by one or a few genes (simply inherited), whereas
others are governed by many genes with small effects (quantitative traits).
The underlying principle in plant breeding is that the phenotype (what is observed)
is a product of the interaction between the genotype (genetic makeup) of the individual
and its environment. Phenotype can therefore be changed by manipulating either the
genotype (plant breeding) or its environment (agronomy). Before starting a breeding
program, breeders should first ascertain whether adequate heritable variation is available.
Then they should have clear goals or objectives. The specific methods used depend
on the mating system of the plant. Genetic changes may be made in flowering plants
through crossing (hybridization).
Molecular biotechnological procedures are being utilized by plant breeders to accelerate
their breeding programs through the use of molecular markers for effective selection.
Further, breeders are able to import genes from outside of the genus with which they are
working and thereby develop transgenic plants. Problems that once were impossible to
solve with conventional breeding are being tackled with amazing success using molecular
biotechnological tools.
It should be emphasized that conventional and molecular biotechnological methods
of breeding are complementary tools. Geneticists and breeders work together to improve
horticultural plants.