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level of the recurrent parent genome (RPG) as reached in BC7).
Nevertheless, for this, large numbers of MDP and more efficient
marker systems are required. In the above scheme, the screening
of the whole population has to be conducted at least once at the
beginning of each BC generation; therefore, it is essential to spot
the foremost convenient set of markers for the allele(s) of interest.
With the population size running into hundreds or thousands,
such screening can be laborious and expensive. However, this
can be optimised by using an appropriate combination of DNA
markers. With the recent advances in molecular technology, par-
ticularly SNPs, a substantial improvement within the capability
of expeditiously screen larger populations can be achieved (Babu
et al. 2004).
efficiency of
MAS
Computer simulation has provided a robust tool for analysing
the planning and potency of MAS programme. Three different
selection strategies in a marker-assisted background selection
programme, namely two-step, three-step and four-step, which,
were compared by computer simulation in terms of faster
recovery of an outsized proportion of the RPG. The simulations
were based on maize genetic map (
n
= 10) with markers spaced
about 20 cM apart and with the assumption that the target locus
could be scored directly either through phenotype or a marker
completely linked to the target gene. Major conclusions from
this simulation experiment are as follows (Babu et al. 2004):
1. A four-stage sampling strategy that includes (a) select-
ing individuals carrying the target allele; (b) selecting
individuals homozygous for recurrent parent genotype at
loci flanking the target locus; (c) selecting the individuals
homozygous for recurrent parent genotype at the remain-
ing loci on the same chromosome as the target allele;
and (d) selecting one individual that is homozygous for
the recurrent parent genotype at most loci (across whole
genome) among those that remain, is the most efficient
procedure in general.
2. With the four-stage sampling strategy and reasonable
population size (50-100), one can expect to find BC3
progeny with at least 96% RPG with 90% probability. It
would take six generations of traditional backcrossing to
reach this stage, besides the risk of a larger probability of
linkage drag around the target gene.
3. Increasing the number of markers genotyped at each
generation had little effect. Once the threshold of one
marker per 20 cM is reached, additional markers (except
