In this article we will discuss about new techniques used in development of gene mapping.
One of the recent applications of new techniques of molecular biology is the rapid development in molecular markers and gene mapping. The use of DNA-based markers is allowing researchers to mark genes or chromosome regions that are related to genetic traits such as disease resistance or fruit colour.
A particular advantage of such techniques is that complex polygenic traits can be analysed. Mapping techniques can also be used to isolate genes based entirely on their genetic behaviour. Once these have been identified, sequenced and cloned, gene transfer techniques can be used to transfer them to other species.
Various techniques have been used to mark individual characters in segregating populations and construct genetic maps. Initially isozymes were used as markers. Isozymes are enzyme products of particular genes. Individuals which vary in their genetic characteristics are separated by polymorphisms in isozyme banding patterns on gel plates.
Recently, the more precise marker techniques such as RFLPs, RAPDs, AFLPs and VNTRs are being preferred. Restriction Fragment length Polymorphisms (RFLPs) are short fragments of DNA generated by restriction enzymes.
The fragments are separated by electrophoresis and are identified with radioactive probes. Random Amplified Polymorphic DNA (RAPDs) are produced by randomly amplifying DNA sequences using PCR technology and primers.
AFLPs are a related PCR-based technique. Highly polymorphic markers which also depend on PCR technology are VNTRs. These are tandem repeats of short DNA sequences and include minisatellites and microsatellites. The use of RAPDs, AFLPs and VNTRs is much quicker than RFLPs. Using these techniques it is possible to link markers on a genetic map with traits of interest in a particular species.
Once markers are defined, they can be used directly in breeding programs since they allow researchers to predict phenotypes based on the presence or absence of the marker. Alternatively, once genes for particular character traits have been identified by marker techniques, they can be cloned.
Although currently a difficult task, the development of automated systems will likely make such ‘map-based cloning’ fairly routine in the future. Major research efforts are underway to sequence, or partially sequence, all genes in model species such Arabidopsis or rice.
Once this is achieved in a major dicot and monocot species, it will be easier to locate particular genes in other species. Genes that are located together in one species are likely to be similarly located in another species.
Alternatively, a gene which codes for a similar function in a related species may be found by identification of homologous DNA sequences. Although much of the initial research effort has been applied to important vegetable and cereal species, these techniques will have a large impact on fruit tree breeding.
For example, the long juvenile period required by many fruit tree species has made the process of breeding and selection of superior genotypes a long term effort. If a genetic marker is obtained for a trait of interest, plants could be selected at the seedling stage.