Intrachromosomal Crossover Increase with Distal Homeologous Substitution

THE frequency of genetic crossing over has long been known to be subject to several forms of experimental modification. Among these, some of the most erratic have been chromosomal inversions, translocations and substitutions. Changes in crossover or chiasma frequency have been found in chromosomes which are themselves structurally modified (in regions either adjacent to or relatively remote from those where the changes were noted). These modifications have been reported in organisms as diverse as Drosophila , maize, cotton and grasshoppers 1–14 . Probably better known is the heterologous effect which occurs in structurally normal chromosomes when other members of the complement have been rearranged. This has been reported in Drosophila , maize, grasshoppers and rye (reviewed by Ramel 15 and subsequently studied by others 16–24 ).