Sometimes the physical location of the markers is known, and sometimes researcher use relative distances among markers to narrow down a location. In the first case, a physical distance can be used. In the second case, a recombination distance is determined. It is assumed that the markers whose pattern of inheritance is most similar to the pattern of inheritance exhibited by the trait of interest are in closest proximity to the gene or genes responsible for that trait. The degree of ease with which genes affiliated with particular traits can be mapped is a function of the density of available markers and the pattern of inheritance of the trait of interest.
Complex traits are traits that are presumably due to the influence of more than one gene.
Simple traits are traits that show simple Mendelian inheritance.