20 



MUTATION AND PLANT BREEDING 



oC /3 



-I 1- 



I 



oC Q 



Synthetic A 1 



Dt 



oC 



A 



Figure 3. — The steps involved in obtaining the synthetic A b . First step 

 involves crossover isolation of the a'.a complex. The second step involves 

 Dt-induced mutation of a in this complex giving a'-A, the synthetic A b 

 in which A replaces (j. 



would seem to he particularly appropriate, since, according to the 

 mutation hypothesis which attributes the noncrossover alpha occur- 

 rences to mutation of the beta element, there is no reason to expect 

 that the synthetic A b , which carries A instead of beta, should yield 

 alpha derivatives at the same rate that A h does. The data presented 

 in the first two rows of Table 7 indicate that the original and the 



Table 7. — Yield of Alpha Derivatives from A b (alpha:beta) and from its Synthetic 

 Counterpart (alpha :.4) in Heterozvgotes with Recessive a. 



Source 



A b 



gametes 



tested 



Alpha derivatives 



nco 



CO 



Sibs: 



A b sh/a Sh 9,180 1 1 a Sh 



/l b -synthetic/a 12,340 Total of 15 a cases* 



.4 h -synthetic Sh/a sh 36,700 3 a Sh 51 a sh 



"These could not be scored for noncrossover or crossover origin since the parent lacked distal 

 marking. 



synthetic complexes yield alpha derivatives with similar frequencies. 

 Moreover, it is apparent that the noncrossover alpha is a rare occur- 

 rence among the progeny of the synthetic A h (Table 7, third row) 

 just as it is among the progeny of the original A h . The 3 noncrossover 

 cases among a total of 54 alpha derivatives reported here for the 

 synthetic complex is in good agreement with the 10 noncrossovers 

 among a total of 127 alpha cases reported for the original A b (8). From 

 these results, indicating that the substitution of A for beta in the 

 Ecuador complex has little if any effect on the frequency of noncross- 



