Genetic linkage map of the Fruit Fly, Drosophila
melanogaster (2n = 4)
The first gene maps were drawn in 1916 from recombination frequency
data collected by Calvin Bridges (1889-1938), a student of Thomas Hunt
Morgan. Dihybrid crosses between flies typically followed the Mendelian
expectation of independent assortment of genes. However, for some pairs,
fewer recombinant types than expected were obtained. Bridges reasoned that
the recombination frequency might provide an estimate of the physical
distance between the genes on a chromosome. [Bridges called a 1%
recombination frequency 1 centimorgan (cM) in honor of his boss]. By
constructing crosses between flies that differed at three markers, the relatives
order of genes could also be determined. Bridges also realized that the genes
