Geoscience Reference
In-Depth Information
perhaps around the target locus) would not be required.
The frequency of recombination, and not the quantity
of markers, is an additionally significant limiting factor
in reducing linkage drag, which proposes that sampling
larger populations with fewer markers makes more sense
than the reverse.
Recent
advances in
MAS strategies
and genotyping
techniques
Single large-scale MAS
In the single large-scale MAS strat-
egy, the recipient genotypes which were locally well adapted
with good yield characteristics are selected for the MAS to intro-
gress the genes responsible for the target trait to be improved.
The donor parent should be selected in such a way that it should
show the polymorphism to the targeted gene. Then the donor
and recipient parental lines are used for crossing to generate
segregating populations. A genomic region of interest for each
parental line is identified by combining favourable alleles in the
segregating populations (e.g. F
3
families and RILs). Foreground
selection is conducted on these large-scale segregated popula-
tions for the targeted alleles of genomic regions with the help of
molecular markers. It offers a few advantages over other strate-
gies, namely, (1) it is more suitable for gene pyramiding of two
or more cloned genes or QTLs, (2) it assures good allelic vari-
ability for further line development in different environmental
situations and (3) no pressure of selection is required (Babu
et al. 2004).
Pedigree MAS
The pedigree MAS strategy is particularly
suitable to those crops where pedigree of germplasm is avail-
able. These selective genotypes must be characterised at the
molecular level for their effective utilisation in the breed-
ing programmes. At each segregating generation, along with
foreground selection, phenotypic selection is also conducted
to identify the desirable genotypes with gene of interest for a
trait. Then the molecular markers which were closely linked to
the trait of interest can be used to enhance fixation of favour-
able alleles in the next generations (offspring 1 and offspring
2). This MAS strategy was suggested to be most efficient when
conducted on F
2
or F
3
segregating populations (Babu et al.
2004).
Precautions to be followed during MAS
In recent years,
molecular marker technologies such as MAS are found to be
a supplemental technology for traditional breeding strategies,
to achieve genetic gains with greater speed and precision.
Although MAS is currently used more commonly for simply
