Geoscience Reference
In-Depth Information
and evaluated over locations and years and maintained with-
out any genotypic change. They are useful for mapping both
qualitative and quantitative characters. But the main demerit
of such population is that since it involves
in vitro
techniques,
relatively more technical skills are required in comparison with
the development of other mapping populations.
Recombinant inbred lines
Recombinant inbred lines (RILs)
are produced by continuous selfing or sib mating the progeny of
individual members of an F
2
population until complete homozy-
gous is achieved. Once homozygosity is achieved, RILs can be
propagated indefinitely without further segregation. The single-
seed descent method is best suited for developing RILs. The bulk
method and pedigree methods without selection can also be used.
RILs also equalise marker types like DHs, so the genetic segre-
gation ratio for both dominant and co-dominant markers would
be 1:1. Since RILs are immortal population, they can be repli-
cated over locations and years and therefore are of immense value
in mapping QTLs. RILs, being obtained after several cycles of
meiosis, are very useful in identifying tightly linked markers.
The major disadvantage associated with this population is that
it requires many seasons/generations to develop and is relatively
difficult in crops with high inbreeding depression.
Near-isogenic lines
Near-isogenic lines (NILs) are gener-
ated either by repeated selfing or backcrossing the F
1
plants to
the recurrent parents. NILs developed through backcrossing
are similar to a recurrent parent but for the gene of interest,
while NILs developed through selfing are similar in pair, but
for the gene of interest (however, they differ a lot with respect
to the recurrent parent). The expected segregation ratio of the
markers is 1:1, irrespective of the nature of marker. Like DHs
and RILs, NILs are also 'immortal mapping population'. These
are quite useful in functional genomics. NILs are directly use-
ful only for molecular tagging of the gene concerned, but not
for linkage mapping and require many generations for develop-
ment. Along with these, linkage drag is a potential problem in
constructing NILs, which has to be taken care. Generally, a
larger population size is needed for high-resolution fine map-
ping, but for preliminary genetic mapping studies a population
size of 50-250 individuals can be used.
Identification of
polymorphism
The presence of sufficient polymorphism between selected
parents is essential before the construction of a linkage map.
Therefore, identification of polymorphic DNA markers, which
