c/7, fl — The Application of Genetics to Plants • 139 
generally inherited in sets, plants whose ploidy 
is increased usnally gain full sets of new 
chromosomes. 0\er one-third of domesticated 
species are polyploids.^ (ienerally, crop im- 
pro\ ement due to increased ploid\’ corresponds 
to an o\erall enlargement in plant size; leaxes 
can he broader and thicker \\ ith larger flow ers, 
fruits, or seeds. .A well-known e.xample is the 
cultixated strawberry, which has four times 
more chromosomes than the wild type, and is 
much fleshier. 
.Another technique, called backcrossing, can 
improx e a commercially superior x ariety by lift- 
ing one or more desirable traits from an inferior 
one. Generally, this is accomplished by making a 
series of crosses from the inferior to the superi- 
or plant XX hile selecting for the desired traits in 
each successixe generation. Self-fertilizing the 
last backcrossed generation results in some 
progenx that are homozygous for the genes be- 
ing transferred and that are identical xxith the 
superior xariety in all other respects. Single 
gene resistance to plant pests and disease-caus- 
ing agents has been successfully transferred 
through backcrossing. 
Major constraints on crop 
improvement 
Txx o of the many constraints on crop breed- 
ing are related to genetics. 
Many important traits are determined by several 
genes. 
The genetic bases for improx ements in x'ield 
and other characteristics are not completely 
defined, mainly because most biological traits, 
such as plant height, are caused by the interac- 
tion of numerous genes. Although many— per- 
haps thousands— of genes contribute to quan- 
titatix e traits, much x ariation can be explained 
by a few' genes that haxe major impact on 
the obserxable appearance (phenotype)'^— e.g., 
the height of some genetic dwarx'es in wheat 
can be doubled by a single gene. Many other 
genes contribute to the general health of the 
^W. J. C. Lawrence. Plant Breeding (London: Edward Arnold Ltd., 
1968). 
■•J. \. Thompson, Jr., ".Analysis of Gene Number and Develop- 
ment in Polygenic Systems," Stadler Genetics Symposium 9:63. 
plant (such as resistance to pests and diseases), 
although some of their contributions are small 
and difficult to assess. Fax'orable combinations 
of genes result in plants xx^ell-adapted to par- 
ticular groxving conditions and agronomic prac- 
tices. With thousands of genes in a single plant 
contributing to overall fitness, the possible com- 
binations are almost infinite. 
Most poor combinations of genes are elim- 
inated by selection of the best progeny; initially 
faxorable combinations are preserved and im- 
prox ed. laterally millions of plants may be ex- 
amined each year to find particularly favorable 
genotypes for development into nexv breeding 
stocks. Increasingly sophisticated field testing 
procedures, as xvell as adx anced statistical anal- 
yses, are noxv used to evaluate the success of 
breeding efforts. Oxerall yield is still the most 
important criterion for success, although con- 
siderable care is taken to test stress tolerance, 
pest and disease resistances, mechanical har- 
xestabilitv, and consumer acceptability. Breed- 
ing programs xvith specialized goals often use 
rapid and accurate chemical procedures to 
screen lines and progeny for improvements. 
Because the x igor of the plant depends on the 
interaction of many genes, it has been difficult 
to identify individual genes of physiological 
significance in xvhole plants. As a result, many 
important genes have not been mapped in 
major crop species. There is little doubt that 
breeders xvould select traits like photosynthetic 
efficiency (the ability to convert light to such 
organic compounds as carbohydrates) or miner- 
al uptake if the genes could be identified and 
manipulated in the same xvays that resistance is 
selected for pathogens. 
It is uncertain how much genetic variation for im- 
provement exists. 
Although the world’s germplasm resources 
have not been completely exploited, it has 
become more difficult for breeders to improve 
many of the highly developed varieties now in 
use— e.g., height reduction in wheat has made 
enormous contributions to its productivity, but 
further improvement on this basis seems to be 
limited.® A parallel condition in the potato crop 
®N. F. Jensen, "Limits to Growth in World Food Production," Sci- 
ence 201:317, 1978. 
