160 



The amount of crossing-over be- 

 tween the knob and the interchange 

 point is approximately 39% (table 1), 

 between c and the interchange ap- 

 proximately 33%, between wx and the 

 interchange, 13% (preceding paper). 

 With this information in mind it is 

 possible to analyze the data given in 

 table 3. The data are necessarily few 

 since the ear contained but few ker- 

 nels. The three individuals in class I 

 are clearly non-crossover types. In 

 class II the individuals have resulted 

 from a crossover in region 2, i.e., be- 

 tween c and 'wx. In this case a cross- 

 over in region 2 has not been accom- 

 panied by a crossover in region 1 (be- 

 tween the knob and C) or region 3 

 (between vjx and the interchange). All 

 the individuals in class III had normal 

 chromosomes. Unfortunately, pollen 

 was obtained from only 1 of the 6 in- 

 dividuals examined for the presence of 

 the knob. This one individual was 

 clearly of the type expected to come 

 from a gamete produced through 

 crossing-over in region 2. Class IV is 

 more difficult to analyze. Plants 6, 9, 

 10, 13, and 14 are normal and WxWx; 



CREIGHTON AND MCCLINTOCK 



they therefore represent non-cross- 

 over types. An equal number of non- 

 crossover types are expected among 

 the normal Wxnjox class. Plants 1, 2, 4, 

 1 1 and 12 may be of this type. It is pos- 

 sible but improbable that they have 

 arisen through the union of a c-Wx 

 gamete with a gamete resulting from 

 a double crossover in region 2 and 3. 

 Plants 5 and 8 are single crossovers 

 in region 3, whereas plants 3 and 7 

 probably represent single crossovers 

 in region 2 or 3. 



The foregoing evidence points to 

 the fact that cytological crossing-over 

 occurs and is accompanied by the ex- 

 pected types of genetic crossing-over. 



Co?wb{sio7is.— Pairing chromosomes, 

 heteromorphic in two regions, have 

 been shown to exchange parts at the 

 same time they exchange genes as- 

 signed to these regions. 



The authors wish to express appre- 

 ciation to Dr. L. W. Sharp for aid in 

 the revision of the manuscripts of this 

 and the preceding paper. They are in- 

 debted to Dr. C. R. Burnham for fur- 

 nishing unpublished data and for some 

 of the material studied. 



