Geoscience Reference
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certain traits to be 'fast-tracked', leading to faster line
development and variety release. Markers may be used
to reduce the time for phenotyping that permits selection
in off-season nurseries making it a lot of cost and time
effective to grow more generations per year (Ribaut and
Hoisington 1998).
Another benefit from MAS is that the total number of lines
that need to be phenotypically tested can be reduced early
in the breeding scheme, which permits more efficient use of
glasshouse and/or field space, which is often limited because
only important breeding material is maintained. Despite hav-
ing these potential advantages over conventional breeding,
a marker will not necessarily be useful or more effective for
every trait and they require a substantial investment in time,
money and resources for their development. For many traits,
effective phenotypic screening methods already exist and
these will often be less expensive for selection in large popu-
lations. However, when whole-genome scans are being used,
even these traits can be selected for if the genetic control is
understood.
Foreground selection and background selection
In marker-
assisted selection, molecular markers are increasingly being used
to trace the presence of target genes (foreground selection), fur-
ther so as to speed up the recovery of the recurrent parent genome
(background selection) in backcross programmes. Conventional
backcrossing in plant breeding is employed to introgress favour-
able traits from a donor plant into a recurrent parent. During this
continual crossing procedure, large segment of donor genome
containing some undesirable gene along with target allele addi-
tionally is introduced into recurrent parent genome and recon-
struction of recurrent parent genome needs a minimum of six
backcross generations. So as to minimise this linkage drag,
marker assay is advantageous. A marker is employed in terms of
marker-assisted backcrossing to either mark out the target gene
or to recover the recurrent parent genotype to enhance the effi-
ciency of backcross breeding. Conventional backcross breeding
programmes needs extra selfing generations after every back-
cross generation for the transfer of recessive genes that reduces
the effectiveness of most conventional breeding processes.
Melchinger has effectively used marker-assisted foreground
selection for introgression of disease-resistance genes by present-
ing a priori approach for calculating the minimum range of indi-
viduals and family size needed in recurrent backcrossing. Still,
