Interchromosome Effects of Chiasmata and Crossing-over

Abstract

THE significance of chiasmata in relation to crossing-over inferred by Janssens was more fully recognized with the development of the partial chiasmatypy theory by Darlington¹. Although in recent years mechanisms of crossing-over, very different from that based on the breakage and reunion hypothesis, have been proposed, a direct relationship between chiasmata and genetic recombination of the reciprocal type characteristic of most organisms has continued to be valid. Thus, as indicated by Pritchard², chiasma formation would be expected to occur regularly even on the copy ing-choice model of Lederberg³ so long as the two copies are synchronized in their switching from one template to another, as they are expected to be, in the case of reciprocal exchanges. Whatever may be the mechanism of crossing-over—and it is widely recognized that the copy choice model as well as the mechanism proposed by Belling, though attractive, present considerable difficulties with regard to their wider applicability^4,5—further evidence, briefly described here, has become available in recent years which either shows a direct correlation between chiasma formation and chromatid exchange or indicates in other ways a parallelism between chiasmata and crossing-over.