Geoscience Reference
In-Depth Information
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Screening of polymorphism between parents and segregants
•
Linkage analysis to determine gene order and map dis-
tance with the help of different statistical tools
Mapping
populations
A population used for gene mapping is commonly called a
mapping population. The commonly used mapping popula-
tions are obtained from controlled crosses. The selection of a
population for genome mapping involves choosing parents and
determining a mating scheme. Decisions on the selection of
parents, mating designs and the type of markers depend upon
the objectives of the experiments. Parents of mapping popula-
tions must have sufficient variation for the traits of interest at
both the DNA sequence and phenotypic level. The variation at
the DNA level is essential to trace the recombination events.
The more DNA sequence variation exists, the easier it is to find
polymorphic informative markers.
The selection of parents for developing a mapping popula-
tion is critical to successful map construction. Since a map's
economic significance will depend upon marker-trait asso-
ciation, as many qualitatively inherited morphological traits
as possible should be included in the genetic stocks chosen
as parents for generating the mapping population. Diagrams
representing different types of mapping populations are
shown in Figure 17.2. Different types of mapping popula-
tions that are most frequently used in linkage mapping are
as follows:
F
2
population
Such populations are produced by selfing or
sib mating the individuals in segregating populations generated
by crossing the selected parents. The expected ratio of domi-
nant and co-dominant markers from F
2
population is 3:1 and
1:2:1, respectively. These are considered to be the best popula-
tion for preliminary mapping and require minimum effort and
time for development. Such populations are temporary popula-
tions because they are highly heterozygous and cannot be prop-
agated indefinitely through seeds, and therefore, are of limited
use for fine mapping.
F
2
derived F
3
(F
2:3
) population
F
2:3
populations are obtained
by selfing the F
2
individuals for a single generation. Such popu-
lations are suitable for specific situations like mapping quan-
titative traits and recessive genes. The F
2:3
families can be
used for reconstituting the genotype of respective F
2
plants, if
needed, by pooling the DNA from plants in the family. Like F
2
population, it is not 'immortal'.
