12 



MUTATION AND PLANT BREEDING 



Table 3. — Constitutions of Alpha-bearing Strands from A h ~P/A h (Ecuador) 



Individuals Marked with T and sh. 



whose members have retained synaptic homology. 



Mention was made at an earlier point of the possibility that 

 the nonrecombinant alpha cases might represent double exchanges 

 in which one crossover occurs between the alpha and beta 

 elements and another within the marked segment such as to 

 reconstitute a parental marker combination. Critical evidence is 

 provided on this point by an analysis of A h -V hemizygotes in which 

 the homolooue is deficient for a seoment including the A locus. If 

 both the recombinant (co-1) and nonrecombinant (nco— 1) alpha 

 derivatives from A h -P /a parents (Table 1) are dependent on a cross- 

 over between the beta and alpha elements, it is expected that ^'-P/Df 

 a-X individuals in which the opportunity for synapsis of the A h -P 

 complex and hence for exchanges between homologues in that region 

 is removed, would yield no alpha offspring. 



The data summarized in Table 4 indicate that alpha occurrences 

 are common among the offspring of such hemizygotes. The deficien- 

 cies rt-Xl and a-X3 are of X-ray origin (23) and are known to be 

 deficient for a segment including the A locus and extending to the 

 right beyond the Sh locus. That the deficiency extends to the left 

 beyond beta in the homologue is apparent from the absence of A 7 a Sh 

 recombinants (co-1) among the alpha strands from Y'-marked hemi- 

 zygotes. (See first four rows of Table 4.) Their complete absence is 

 somewhat surprising since they might be expected as a result of 

 coincidental exchange in the T - ft segment. However, this coin- 

 cidental event may be rarer than anticipated either as a result of an 

 interfering effect of the event that gives rise to alpha, or because the 



