204 



Miscellaneous Linkage Topics 



homologous ends are not paired. The normal knobless chromo- 

 some had the genes c and Wx, whereas the interchange knobbed 

 chromosomes had C and wx (Table 5). The original cross was 



Jl 



a b c d e f 



Fig. 61. Chromosomes involved in Creighton and McClintock's demon- 

 stration of crossing over: (a) a normal knobbed chromosome IX; (b) a 

 normal knobless chromosome IX; (c) a normal chromosome VIII. By a 

 reciprocal translocation, a piece of chromosome VIII exchanged with a 

 piece of chromosome IX of the knobbed strain, producing an interchange 

 chromosome IX (d) and an interchange VIII (e). In a plant with a nor- 

 mal knobless IX, a normal VIII, and the two interchange chromosomes, 

 a quadrivalent figure would be found as at (/). 



made, then, in the repulsion phase, and the Fi was crossed with 

 a plant that had two normal knobless chromosomes and the 

 genes c Wx / c wx. If only the c icx chromosome is considered, 

 this represents a testcross. If a crossover occurred between the 

 c and wx loci of the Fi, the parental types and nonparental 

 types should be: 



Phenotypes Chromosomes 



Parental types: colorless; starchy knobless; normal 

 colored; waxy knobbed; long 



Recombinations: colorless; waxy 

 colored; starchy 



knobless; long 

 knobbed; normal 



All four types were recovered. The phenotypes showed that 

 genetic crossing over had occurred between the c and ivx loci. 

 Cytological observations of the heteromorphic bivalent showed 

 that a break and realignment had occurred between the knobbed 

 end and the interchanged end. The correlation between the cy- 

 tological observations and the genetic results afforded proof of 



