Cytological Map

a schematic representation of chromosomes indicating the arrangement of individual genes, obtained by cytological methods.

Cytological maps are prepared for organisms for which genetic chromosome maps already exist. Every site of a gene (locus) on a genetic map, established on the basis of the frequency of crossing-over of portions of a chromosome, is linked on a cytological map to a specific, actually existing chromosome portion. This is one of the main pieces of evidence in favor of the chromosome theory of heredity. Cytological maps are constructed on the basis of an analysis of chromosomal aberrations, for example, nondisjunctions and deletions. The site of a particular gene in a chromosome can then be determined by comparing the changes in the morphological characters of the chromosomes in the aberrations and the changes in the genetic properties of the organism. Using the method of chromosomal aberrations, the American geneticist C. Bridges in 1935 prepared a detailed cytological map for the fruit fly Drosophila, the best studied organism from the genetic point of view.

The giant chromosomes of insects of the order Diptera proved to be the most convenient for constructing cytological maps, since in addition to their large size, they possess a distinct morphological configuration: each portion of these chromosomes has its own distinct markings, produced by the characteristic longitudinal alternation of brightly staining portions (disks) and weakly staining portions (interdisks). The use of cytological methods facilitates the determination of the absence of a chromosome portion or the transfer of the portion elsewhere. A comparison of the cytological maps with the genetic maps showed that the physical distance between the genes in the chromosomes does not correspond to the genetic distance (the frequency of crossing-over apparently differs from one part of a chromosome to another). Consequently, the density of gene distribution on cytological and genetic maps differs. An important genetic phenomenon—the unequal frequency of crossing-over along the length of a chromosome—was thus established.

The linear arrangement and sequence of genes established by genetic methods are confirmed by cytological maps. Modern cytological and genetic methods make it possible to construct cytological maps of many organisms, including man.