laughnan: nature of mutations 23 



Strand Types Isolated 



Event- I Event- 2 



■c Sh T «c Sh 



T flfi*. Sh T jajz Sh 



i i — i — i 1— ' i — i i 





 O 



Figure 4. — Diagrammatic representation of proposed mechanism of auto- 

 association. At the left, adjacent, homologous beta and alpha segments of 

 a single chromosome are shown paired zvith each other at meiosis to form 

 the double loop. The results of exchange event 1 (involving sister chroma- 

 tids) and of exchange event 2 (involving a single chromatid) are shown at 

 the right. Both events yield a "noncrossover" alpha strand, but the first 

 event produces a complementary strand carrying two beta members 

 plus an alpha, while the second event yields a parental-type strand 

 carrying beta and alpha members plus an acentric ring representing the 

 beta member. 



are explained. The observation that loss of beta is conditioned by 

 the presence of alpha in the parental complex is also predicted since, 

 on the model proposed here, the removal of the beta element is 

 dependent on the adjacent alpha-carrying segment to provide the 

 double loop. Finally, since the A.A. hypothesis involves a strictly 

 intrachromosomal event, the occurrence of noncrossover alpha cases 

 among the progeny of plants heterozygous for the a-X deficiencies 

 is also anticipated. 



In fact, assuming that the A.A. type of pairing at meiosis is in 

 competition with conventional interhomologue pairing, the fre- 

 quency of the former, and therefore of noncrossover alpha occur- 

 rences, should be enhanced in the deficiency heterozygote, in which 

 the opportunity for interhomologue pairing of members of the com- 

 plex is removed. Results of experiments testing this prediction will 

 be published elsewhere, but it may be mentioned here that studies 

 of this type dealing with over 400 alpha cases and with tests of over a 

 million gametes indicate a significant enhancement of the alpha rate 

 in the hemizygote. 



The possibility that sister strand crossing over might account 

 for the origin of the noncrossover alpha derivatives was treated in 

 an earlier publication (8) but was not considered a likely explanation 

 because (a) there was evidence against the occurrence of the phe- 



