Linkage: from particulate to interactive genetics

Genetics was established on a strictparticulate conception of heredity. Geneticlinkage, the deviation from independentsegregation of Mendelian factors, was conceivedas a function of the material allocation of thefactors to the chromosomes, rather than to themultiple effects (pleiotropy) of discretefactors. Although linkage maps wereabstractions they provided strong support forthe chromosomal theory of inheritance. DirectCytogenetic evidence was scarce until X-rayinduced major chromosomal rearrangementsallowed direct correlation of genetic andcytological rearrangements. Only with thediscovery of the polytenic giant chromosomes inDrosophila larvae in the 1930s were thevirtual maps backed up by physical maps of thegenetic loci. Genetic linkage became a pivotalexperimental tool for the examination of theintegration of genetic functions in developmentand in evolution. Genetic mapping has remaineda hallmark of genetic analysis. The location ofgenes in DNA is a modern extension of thenotion of genetic linkage.