ABSTRACT
Since the rediscovery of Mendelian principles, the field of genetics has witnessed many landmarks in understanding genetic principles. Better understanding of genetic principles using molecular approaches led to a new subdivision of genetics called ‘Genomics’. The development and abundant availability of molecular markers such as restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism
(AFLP), sequence tagged site (STS), microsatellites or simple sequence repeats (SSRs) and sequence based markers such as single nucleotide polymorphism (SNP) during the last two decades has opened up new avenues to locate and measure the effects of the gene(s) in an unprecedented manner. Molecular markers are now widely used to track loci and genomic regions in plant breeding programs. Genetic maps are built based on the meiotic recombination between homologous chromosomes. Though the recombination has been used to order the genes, the application of molecular markers made significant impact to developing saturated linkage map, which is one prerequisite for map-based gene cloning. On the other hand, physical mapping reflects the actual physical distance in base pairs between molecular markers, which is becoming increasingly important to understand the molecular insights of the genes. In principle, genetic and physical maps should have the same order of markers and genes and equivalent distances among the loci. However, the genetic distances in recombination units (cM) and the physical distances (bp) may not be the same due to uneven distribution of crossing-over events along chromosome arms. Genetic and physical mapping are the components of forward genetic approach, in which the ultimate goal is to physically locate the gene(s) of interest through highdensity markers using large segregating mapping population(s).
