138 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
breeding objectives are specific responses to the 
needs of local growers, to consumer demands, 
and to the requirements of the food processing 
firms and marketing systems. 
Developing new varieties does the farmer lit- 
tle good unless they can be integrated profitably 
into the farming system either by increasing 
yields and the quality of crops or by keeping 
costs down. The three major goals of crop 
breeding are often interrelated. They are: 
• to maintain or increase yields by selecting 
varieties for: 
—pest (disease) resistance; 
—drought resistance; 
—increased response to fertilizers; and 
—tolerance to adverse soil conditions. 
• to increase the value of the yield by select- 
ing varieties with such traits as: 
—increased oil content; 
—improved storage qualities; 
—improved milling and baking qualities; 
and 
—increased nutritional value, such as high- 
er levels of proteins. 
• to reduce production costs by selecting 
varieties that: 
—can be mechanically harvested, reducing 
labor requirements; 
—require fewer chemical protectants or 
fertilizers; and 
—can be used with minimum tillage sys- 
tems, conserving fuel or labor by reduc- 
ing the number of cultivation operations. 
The plant breeder's approach to 
commercialization of new varieties 
The commercialization of new varieties 
strongly depends on the genetic variability that 
can be selected and evaluated. A typical plant 
breeding system consists of six basic steps: 
1. Selecting the crop to be bred. 
2. Identifying the breeding goal. 
3. Choosing the methodological approach 
needed to reach that goal. 
4. Exchanging genetic material by breeding. 
5. Evaluating the resulting strain under field 
conditions, and correcting any deficiencies 
in meeting the breeding goal. 
6. Producing the seed for distribution to the 
farmer. 
The responsibilities for the different breeding 
phases are distributed but interactive. In the 
United States, responsibility for crop impro\ e- 
ment through plant breeding is shared by the 
Federal and State governments, commercial 
firms, and foundations.^ Although some specific 
genes have been identified for breeding pro- 
grams, most improvements are due to gradual 
selection for favorable combinations of genes in 
superior lines. The ability to select promising 
lines is often more of an art (in\'ol\ ing years of 
experience and intuition) than a science. 
The plant breeder’s approach is detei’iiiined 
for the most part by the particular biological 
characteristics of the crop being bred— e.g., the 
breeder may choose to use a system of inbreed- 
ing or outbreeding, or the two in combination, 
as an approach to controlling and manipulating 
genetic variability. The choice is influenced l)v 
whether a particular plant in question naturally 
fertilizes itself or is fertilized by a neighboring 
plant. To a lesser degree, the breeding objec- 
tives influence the choice of methods and the se- 
quence of breeding procedures. 
Repeated cycles of self-fertilization leduce 
the heterozygosity in a plant, so that after nu- 
merous generations, the breeder has homozy- 
gous, pure lines that breed true. (S(?e Ti'ch. Note 
4, p. 162.) Cross-fertilization, on the* oth(>r hand, 
results in a new mixture of genes or increased 
genetic variability. Using these two ap|)i()aches 
in combination produces a hybrid— scnci-al lin«>s 
are inbred for homozygosity and tlu'ii ci-ossed 
to produce a parental line of enhanc(>d gencMie 
potential. More vigorous hybrids can he se- 
lected for further testing, fhe (dfeets of hyl)rid 
vigor vary and include earlier gei niination, in- 
creased growth rate or size;, and grc’ater ci'op 
uniformity. 
A second method for exchanging or adding 
genes is achieved through altering the number 
of chromosomes, or ploidy (s(‘e I'eeh ,\oi«* .') |) 
162.), of the plant. Sinc(? chromosomes are 
^Natioriiil AcacifMiiv of Sciences, (V»n.ser\.'ifi()/i iit f,c/ m/i/.iMn /), 
sources: An Imperative, U ashinulon I) ( I!I7K 
