72 M. M. RHOADES 



frequencies of F2 phenotypes can be compared with those calculated on the 

 assumption of independent assortment. The two values agreed very closely, 

 indicating little or no deviation from the law of independent assortment. 

 His data, from plants where loci in chromosomes 9 and 6 are both segregat- 

 ing preferentially, likewise permit such a conclusion to be drawn. 



In my 1942 paper on preferential segregation the statement was made 

 that the chromosomes in plants with the abnormal chromosome 10 formed 

 extra chromosomal (half spindle) iibers at regions other than the true centro- 

 mere region. Rhoades and Vilkomerson (1942) found these supernumerary 

 chromosomal fibers were produced only in plants homozygous or heterozy- 

 gous for the abnormal 10, and that sister plants homozygous for the normal 

 10 had chromosomal fibers originating solely from the localized centric re- 

 gion in an orthodox manner (see Fig. 8 of Plate II). Although the abnormal 

 chromosome 10 was clearly responsible for the formation of these neo-centric 

 regions, they were not restricted to this chromosome since many of the non- 

 homologous chromosomes had supernumerary chromosomal fibers. The ab- 

 normal chromosome 10 is thus responsible for the formation of neo-centric 

 regions, as well as for preferential segregation. Since 1942, a considerable 

 body of data has been obtained bearing on the behavior of abnormal 10. 

 Some of the more pertinent observations have suggested a cytological mecha- 

 nism for the phenomenon of preferential segregation. 



The unorthodox formation of supernumerary chromosomal fibers from neo- 

 centric regions is limited to the two meiotic divisions. (For a description of 

 normal meiosis in maize see Rhoades, 1950.) The first meiotic division is in 

 no way exceptional until metaphase I is reached. Normal appearing bivalents 

 are co-oriented on the spindle figure in a regular manner with the half spindle 

 fibers, arising from the true centric regions, extending poleward. Normally 

 these fibers effect the anaphase movement of the disjoining dyads with the 

 localized centromere region leading the journey to the spindle pole. How- 

 ever, in plants with the abnormal 10, chromosomal fibers arise from distal 

 regions of the chromosome while the bivalents are still co-oriented on the 

 spindle at metaphase I. The neo-centric regions are drawn poleward more 

 rapidly than the true centric regions. Consequently the distal ends, instead 

 of being directed toward the spindle plate during anaphase I, lead the way 

 to the pole. 



The appearance of many disjoining dyads at anaphase I suggests that 

 their poleward migration is due largely, even exclusively, to the fibers origi- 

 nating from the neo-centric regions. The primary centric region appears to 

 play no active role even though it possessed chromosomal fibers at meta- 

 phase I when the tetrad (bivalent) was co-oriented. At mid-anaphase there is 

 no indication of the presence of these fibers in many of the dyads with the 

 precocious neo-centric regions. 



Figure 4.5 and Figures 3 and 4 of Plate I illustrate some of the observed 



