THE PRINCIPLES OF HEREDITY 



513 



chromatids of opposite chromosomes sometimes trade places. This is 

 a process known as crossing-over, and through it a group of genes on 

 one portion of the chromosome may become joined to genes lying on the 

 opposite chromosome. In this way the recombinations of genes can 

 take place. In the Drosophila cross we found about 6 per cent of the 



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From Genetics, Winchester, Houghton Mifflin 



Fig. 32.6. Method of chromosome crossing over during the first division of meiosis. 

 The paired chromosomes (1) become duplicated (2) and there are now four chroma- 

 tids. Crossing over takes place (3) when one chromatid of each chromosome breaks 

 and becomes reattached to a portion of a chromatid of the opposite chromosome. 

 These pull apart in the anaphase (4). The last figure (5) shows the four chromo- 

 somes after the second division of meiosis. 



flies in the second generation represented such recombinations. This num- 

 ber will vary in accordance with the distance between the genes on the 

 chromosome, since there will be a greater chance for crossing over be- 

 tween genes lying far apart than for those lying close together. Because 



