Abstract Mapping started exactly 50 years ago when Bell & Haldane (Proc. R. Soc. Lond. 123, 119 (1937)) measured the genetic distance between colour blindness and haemophilia. In their Discussion they wrote ‘if...an equally close linkage were found between the genes determining blood group membership and that determining Huntington’s chorea, we should be able, in many cases, to predict which children of an affected person would develop this disease, and to advise on the desirability or otherwise of their marriage’. Progress in this direction has proceeded through the discovery of autosomal linkages by family studies, and the assignment of genes to particular chromosomes by somatic-cell hybridization techniques. Recombinant DNA technology has been successfully used in both approaches, with the result that many chromosomes are now roughly mapped. In practice, the map can already be used for prenatal diagnosis of several diseases, and may provide ‘take-off’ points for some molecular approaches to poorly defined genes. More fundamentally, it is beginning to provide insights into the nature of the meiotic process and the organization of the genome.