Genetics and Plant Breeding - An Introduction to Plant Breeding - page 60

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Genetics and Plant Breeding

INTRODUCTION

Molecular genetics , where studies are carried out at
the molecular level. Molecular techniques have been
developed to investigate and handle both qualitative
and quantitative characters. Although the details of
molecular genetics are generally outside the scope of
this topic, the impact of molecular genetic techniques
is important and relevant

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Early plant breeders, basically farmers, did not have any
knowledge of the inheritance of characters in which they
were interested. The only knowledge they possessed was
that the most productive offspring tended to originate
from the most productive plants and that the better
flavour types tended to be derived from parent plants
which were, themselves, of better flavour. Nonetheless,
the achievements of these breeders were considerable
and should never be underestimated. They moulded
most of the crops as we recognize them today from their
wild and weedy ancestral types.
In modern plant breeding schemes it is recognized,
however, that it is very much more effective and efficient
(or indeed essential) to have a basic knowledge of the
inheritance or genetics of traits for which selection is to
be carried out.
There are generally five different areas of genetics that
have been applied to plant breeding:

QUALITATIVE GENETICS

Few sciences have as clear-cut a beginning as modern
genetics. As mentioned previously, early plant breed-
ers were aware of some associations between parent
plant and offspring and at various times in history
researchers had carried out experiments to study such
associations. However, experimental genetics with real
meaning began in the middle of the 19th century with
the work of Gregor Johann Mendel, and only fully
appreciated after the turn of the 20th century.
Mendel's definitive experiments were carried out in a
monastery garden on pea lines. The flowers of pea plants
are so constructed as to favour self-pollination and as a
result the majority of lines used by Mendel were either
homozygous or near-homozygous genotypes. Mendel's
choice of peas as an experimental plant species offered
a tremendous advantage over many other plant species
he might have chosen. The differences in characters
he chose were also fortuitous. Therefore many present
day scientists have argued that Mendel had a great
deal of luck associated with his findings because of the
choices he made over what to study. When this is com-
bined with segregation ratios that are better than might
be expected by chance, many have concluded that he
must 'have already foreseen the results he expected to
obtain'.

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Qualitative genetics , where inheritance is controlled
by alleles at a single locus, or at very few loci

Population genetics , which deals with the behaviour
(or frequency) of alleles in populations and the con-
ditions under which they remain in equilibrium or
change. Thus allowing predictions to be made about
the properties and changes expected in populations

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Quantitative genetics , for traits where the variation
is determined by alleles at more than a few loci, traits
that are said to be controlled by polygenic systems.
Quantitative genetics is concerned to describe the
variation present in terms of statistical parameters
such as progeny means, variances and covariances

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Cytogenetics , the study of the behaviour and proper-
ties of chromosomes being the structural units which
carry the genes that govern expression of all the traits