Mapping the chromosomes—that is, defining the locus of the genetic elements responsible for particular phenomatypes to specific chromosomal sites—was a first step in connecting phene to gene. The mapping process began for the autosomes in 196827 and advanced rapidly in the next 20 years (fig. 4 ). (In 1968, as indicated by the second edition of MIM published that year, 68 phenotypes, judged to be clearly X linked and therefore honored with an asterisk, had been mapped to the X chromosome, but the regional localization on the X chromosome had not yet been established for any of them.)
Figure 4 Growth of information in MIM concerning mapping of genes and genetic loci to specific human chromosomes in the period up to the initiation of the Human Genome (Sequencing) Project. In 1968, when the first gene was assigned to a specific autosome (the Duffy blood group to the centromeric region of chromosome 1),27 MIM recorded 68 X-linked phenotypes with an asterisk, indicating confidence in X-linked inheritance.