Geoscience Reference
In-Depth Information
segment is more than 20 cM) to eliminate the chance of
genotypes presenting a double recombination between
the two flanking markers. Depending on the character of
the genomic region (cloned factor, major or minor QTL)
involved within the expression of a target trait and the
range of selected QTLs or genomic regions that require
to be manipulated, many MAS schemes have been pro-
posed that will be discussed later.
In the context of MAS, DNA-based markers may be effec-
tively utilised for two basic purposes: (i) identifying favourable
allele(s) (dominant or recessive) across generations and (ii) trac-
ing the most appropriate individual(s) among the segregating
progeny, based on allelic composition across a part of or the
whole genome. The fundamental advantages of MAS over con-
ventional phenotypic selection are as follows:
•
Target alleles that are difficult to score phenotypically
and environmentally sensitive and cost and time ineffec-
tive is selected with the assistance of markers. Therefore,
marker-assisted selection is easier than phenotypic selec-
tion or screening procedure. Classical examples of traits
that are difficult and laborious to measure are cereal cyst
nematode and root lesion nematode resistance in wheat
(Eagles et al. 2001; Eastwood et al. 1991; Zwart et al.
2004). Other examples are quality traits that usually need
expensive screening procedures.
•
DNA markers enable early selection for traits that are
expressed in later developmental stages because plants
can be screened early at the seedling stage or as seeds.
Thus, undesirable plant genotypes are quickly elimi-
nated. This might have tremendous advantages in rice
breeding as a result of typical rice production practices
that involve sowing pre-germinated seeds and transplant-
ing seedlings into rice paddies, making it easy to trans-
plant solely selected seedlings to the main field.
•
Many traits that could not be selected on single plant basis
by conventional phenotypic screening strategies because
of environmental errors are selected with the help of
molecular markers. Co-dominant markers in MAS will
discriminate between homo and heterozygous plants for
a few traits by individual plant selection that is not pos-
sible by conventional phenotypic screening methodology.
These advantages of MAS can be exploited by breeders
to accelerate the breeding method. This might facilitate
