atively high individual effects of a single QTL were found. Also, the
total phenotypic variation of a given trait that could be accounted for by
combined assessment of these QTLs was in the range of 50% or higher,
which demonstrates the potential of molecular markers as a selection
tool for tracing quantitatively inherited traits.
Limitations of MAS relate to the fact that applicability of these mark-
ers often depends heavily on the specific experimental population in
which they were identified, as well as on the extent of marker poly-
morphism, linkage disequilibria and linkage phases among the individ-
uals under selection. In addition, specific marker technology and
know-how is required for routine marker-assisted selection programs,
rendering the cost efficiency of MAS uncertain for crops that are not of
major economic importance.
As a more recent achievement, transformation methods enabling the
transfer of any isolated gene into virtually any important cultivated plant
species have opened the toolbox of genetic engineering as a novel op-
portunity to the plant breeder. Plant transformation technology allows
breeders to circumvent some restrictions of classical breeding methods.
In particular, gene technology offers the breeder the following promises:
- The manipulation of native and the introduction of foreign specific
genes not only allow for modification of simply inherited traits like
herbicide tolerance and virus and insect resistance, they also open
new horizons for directed adjustment of even traits that display very
complex inheritance in native systems. Thus, in some instances ap-
proaches of factorial instead of quantitative genetics may be suffi-
cient for the breeder to cope with the improvement of specific quality-
or even yield-determining characters. - Genetic engineering provides “added value” by transferring specific
genes to cultivars that have been subject to intensive breeding efforts
and, thus, are highbred already with respect to yield, uniformity, dis-
ease resistance, and so on. This aspect cannot be underestimated since
any modern cultivar is defined by a complex of characters that has
been assembled during years or decades of breeding work. In classi-
cal breeding new or better characters have to be introduced by crosses,
giving rise to sexual recombination and, as a consequence, to dis-
persion of the valuable trait complexes in the progeny. In addition,
undesired genes from the donor cross parent are also introduced,
which have to be eliminated by successive backcrosses to the culti-
var parent. It is for this reason that plant breeders tend to prevent the
Biotechnology and Plant Breeding 23