Geoscience Reference
In-Depth Information
show significant differences between parents, is the second
most important step in the construction of a linkage map. DNA
polymorphism levels are generally found to be less in inbreed-
ing species as compared to cross-pollinated species; therefore,
selection of distantly related parents is useful in inbreed-
ing species. Once polymorphic markers have been identified
between parents, they must be used to genotype the entire map-
ping population. This is known as marker 'genotyping' of the
population. Different molecular markers like RFLPs, SSRs,
ESTs, CAPs, RAPD, AFLP, ISSR, DArT and SNPs are used
for the construction of a linkage map in several plants. Each
marker system has several advantages and disadvantages, but
generally, highly reproducible, high-throughput, co-dominant
and transferable molecular markers are used to increase the
utility of genetic maps. The expected segregation ratios for co-
dominant and dominant markers are presented in Table 17.2.
Remarkable deviations from expected ratios can be analysed
using chi-square tests.
Generally, markers segregate in a Mendelian fashion even
though distorted segregation ratios can be encountered. Significant
deviation from expected segregation ratio in a given marker/
population combination is referred to as segregation distor-
tion. There are several reasons for segregation distortion,
including: gamete/zygote lethality, meiotic drive/preferential
segregation, sampling/selection during population develop-
ment and differential responses of parental lines to tissue
culture in case of DHs. In some polyploidy species such as
sugarcane, identifying a polymorphic marker is more compli-
cated. The mapping of diploid relatives of polyploidy species
may be of great benefit in developing maps for polyploidy
species. However, diploid relatives do not exist for all poly-
ploidy species. Generally, mapping of polyploidy species is
based on the use of single-dose restriction fragments (Ripol
et al. 1999; Wu et al. 1992).
table 17.2
Expected segregation ratios for markers in
different population types
Co-dominant
markers
Dominant
markers
Population type
F
2
1:2:1 (AA:Aa:aa)
3:1 (B:bb)
Backcross
1:1 (Cc:cc)
1:1 (Dd:dd)
Recombinant inbred or
doubled haploid
1:1 (EE:ee)
1:1 (FF:ff)
