THE ORIENTATION OF MULTIPLE ASSOCIATIONS RESULTING FROM INTERCHANGE HETEROZYGOSITY

Abstract

Among the many factors thought to influence the orientation associations in interchange heterozygotes, previous evidence suggests that there is a genetically controlled mechanism (or mechanisms) which is distinct from that varying the position and number of chiasmata. In rye, variation between 50% and 90% disjunction frequency has been ascribed to this mechanism. Nevertheless, in this species, directed segregation has been found in newly arising interchanges, suggesting that the genetic constitution has been selected to give a high disjunction frequency. Two alternative explanations for these findings were considered. First, the influence on orientation may be a secondary effect of factors whose primary effect is of selective importance in normal, non- interchange individuals. Second, direct selection for a high disjunction frequency may have occurred in a manner analogous to Fisher's theory of the evolution of dominance. (auth)