Cytogenetic Proof of Crossing Over 203 



of differentiating visually the maternal segments from the pa- 

 ternal. 



In 1931, the demonstration that genetic crossing over is cor- 

 related cytologically with an exchange of pieces of homologous 

 chromatids was made independently by Creighton and McClin- 

 tock in maize and by Stern in Drosophila, and the methods of 

 demonstrating it were the same in each organism. 



Since two homologous chromosomes are normally indistin- 

 guishable cytologically, the object of the method was to find or 

 create chromosomes in which the two ends of two homologues 

 were visibly different, while the central part, in which there were 

 known genes whose phenotypic effects could be observed, was 

 the same. In maize, chromosome IX was studied. It bears the 

 genes C and c for colored and colorless aleurone and Wx and wx 

 for starchy and waxy endosperm. Some strains of maize have a 

 chromosome IX in which a large "knob" of genetically inert 

 heterochromatin is present at the end towards which gene C lies; 

 other strains lack the knob. Otherwise identical, the two ho- 

 mologues can be distinguished in the heterozygote only at the 

 end with the knob; the other end is alike in all strains so that 

 a crossover between two such heteromorphic chromosomes can- 

 not be demonstrated cytologically. Occasionally, however, ab- 

 normalities in meiosis occur by which a piece of one chromo- 

 some can become exchanged with a piece of a nonhomologous 

 chromosome. This segmental interchange, or reciprocal trans- 

 location, occurs very rarely in maize under supposedly normal 

 conditions and may be produced by X-rays or radium. 



Creighton and McClintock found a strain of maize in which 

 a piece of a chromosome IX from a knobbed strain had changed 

 places with a larger segment of chromosome VIII at the end 

 away from the knob. These new chromosomes are known as 

 ''interchange" chromosomes. As Fig. 61 shows, this exchange 

 produced a knobbed chromosome IX which was longer than the 

 normal, and as a result could be distinguished at both ends 

 from a normal chromosome IX of the knobless strain. When a 

 normal knobless strain was crossed with an interchanged knobbed 

 strain, the offspring had one normal knobless chromosome IX 

 and one interchanged and knobbed chromosome IX. Since only 

 homologous parts of homologous chromosomes pair, the non- 



