Geoscience Reference
In-Depth Information
map determines not only the position of various genes but also
their relative distances along the chromosome. The basic prin-
ciple behind linkage mapping is the close association of trait of
interest along with marker that could be screened in the map-
ping population formed by mating between the parents P1 and
P2. The schematic diagram representing recombination event
is illustrated in Figure 17.1. The determination of tight associa-
tion between a gene of interest and the marker could be carried
out by knowing the recombination frequency between them that
can be traced with the help of recombinant genotypes recovered
in the segregating generations. A minimum recombination fre-
quency (<10 cM) depicts tight linkage.
The main steps involved in the linkage map construction are:
•
Construction of mapping population and deciding the
sample size
•
Selection of molecular markers for genotyping the map-
ping population
GametesFrequency
A
B
P
45
A
B
R
5
a
A
b
B
a
B
b
R
5
a
b
P
45
a
b
C
D
P
30
C
D
d
20
R
C
c
D
C
d
c
D
R
20
c
d
P
30
c
d
FIGURe 17.1
Diagram indicating cross-over or recombina-
tion events between homologous chromosomes. Gametes
that are produced after meiosis are either parental (P) or
recombinant (R). The smaller the distance between two
markers, the smaller the chance of recombination occurring
between the two markers. Therefore, recombination between
markers C and D should occur more frequently than recom-
bination between markers A and B. This can be observed in
a segregating mapping population. By analysing the number
of recombinants in a population, it could be determined that
markers A and B are closer together compared to C and D.
