N RELATING genetic data to the morphology of the chromosomes in I maize, the relation of gene loci to the position of the centromere, or region of spindle attachment, is of primary interest and importance. The relation of the genes to each other on the linkage maps is established directly from crossing over percentages, but the physical position of the genes in the chromosomes must necessarily be determined indirectly from both genetical and cytological data, sipce suitable techniques for the

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... sual identification of individual genes are not yet available. Chromosomal alterations that produce both genetical and visible cytological effects may be used to locate the position of the genes in the chromosomes. Of these, the most useful are the translocations, inversions and deletions that produce both phenotypic effects suitable for mapping with the techniques used for genes and visible disturbances of the normal synaptic relations by means of which their location in the chromosome can be determined. This correlation of loci on the genetic maps with specific regions of the chromosomes concurrently establishes the location of the genes with reference to the centromere and other differentiated regions of the chromosome. The locations of the centromeres on the linkage maps were determined chiefly from translocations which were observed cytologically at the midprophase of the first meiotic mitosis. The cytological determinations were subject to minor errors of interpretation due to failure of pairing and non-homologous association of the chromosomes in the region of exchange of segments. A single case of duplication, that of a telocentric half-chromosome (RHOADES 1936(RHOADES , 1940) located the centromere of chromosome 5 with a high degree of precision. Deficiencies have been useful in correlating the genetic maps with the distal regions of the chromosomes, less so for the regions near the centromeres. However, the internal deficiencies and ring fragments studied by MCCLINTOCK (1938, 1941) delimited within narrow boundaries certain locations near the centromere of chromosome 5. Translocations may be identified by the sterility they produce when in the heterozygous condition. Thus their position in the linkage maps can be established in much the same manner as are the positions of dominant genes. The