Q.What is a genetic map ?

Because the recombination frequency (RF) between two linked genes is directly proportional to the frequency of crossing over between them, which in turn scales with their physical distance apart on the chromosome, RF data can be used to build a linear genetic (linkage) map showing gene order and inter-gene distances. Alfred Sturtevant, working in Morgan's lab, developed the technique in 1913. Distance is measured in map units (m.u.), where 1 map unit equals 1% recombination frequency; a map unit is also called a centimorgan (cM), and 100 cM make one Morgan (M). A simple two-point test cross (following just two genes) can estimate RF between them, but underestimates true distance because it cannot detect double crossovers, which restore the parental allele combination and so look identical to no crossover at all. A three-point test cross, testing three linked genes together by crossing a triple heterozygote to the triple recessive homozygote, solves this: double crossovers now show up as the two least-frequent progeny classes (because they differ from the parental type only at the middle gene), letting the correct gene order be de …