Chromosome Structure and Crossing Over.

A major obstacle to understanding classical genetic crossing over in higher organisms has been the lack of sufficient knowledge concerning the structure of chromosomes. Without accurate information, in particular, as to how the hereditary substance (DNA) is built into the chromosome, it may be premature to devise models depicting how that material may be recombined in crossing over. In recent years several models of chromosome structure have been proposed (SCHWARTZ 1955,1960; FREESE 1958; TAYLOR l957,1959a, 1962,1963; DE 1964), and the possibilities these offer toward a better understanding of crossing over should now be explored. On the basis of two alternative generalized models of chromosome structure, this paper proposes a new theoretical model of classical crossing over.