this was the first technique used for mapping genes on chromosomes. by working out the number of recombinants it is possible to obtain a measure for the distance between the genes. this distance is called a "genetic map unit" (m.u.), or a "centimorgan" and is defined as the distance between genes for which one product of meiosis in 100 is recombinant. a recombinant frequency (rf) of 1 % is equivalent to 1 m.u. a linkage map is created by finding the map distances between a number of traits that are present on the same chromosome, ideally avoiding significant gaps between traits to avoid the possibility of multiple crossovers.