494 - Heredity and Evolution 



presynaptic CC)OC)CXXXD(1)OC)C)CXXX)OC®OOCCO 



CHROMOSOMES 



SYNAPSIS 



postsynaptic QOOOQQOaBQOOCt MM O«« M « 



CHROMOSOMES iMMtMAMM*XfO^® 



Fig. 26-17. Crossing over. If 

 a cross over occurs at a point 

 that lies between the location of 

 the genes 8 and b and that of 

 the genes V and v, these genes, 

 as the diagram shows, will occur 

 in new combinations in the post- 

 synaptic chromosomes. 



mosomes. The frequency of crossing over— 

 which technically is called the cross-over 

 value— provides a valuable index as to the 

 relative distance between any two pairs of 

 genes having loci on the same pair of chro- 

 mosomes. Cross-over breaks can occur with 

 equal facility along the length of the synaps- 

 ing chromosomes. Consequently, the farther 

 apart are the loci of any two genes, the 

 greater is the likelihood that a cross over 

 will occur between them. And conversely, 

 the nearer together are the loci, the less is 

 the chance of a cross over. Moreover, the 

 cross-over values, expressed on a percentage 

 basis, are additive when one studies a given 

 series of genes; and this proves that the loci 

 are arranged in a specific linear order in each 

 chromosome. These important facts are 

 shown, lor example, by the cross-over values 

 between the genes in chromosomes I of 

 Drosophila, which may be summarized as 

 follows: 



• 7.5 



Crossing over between two consecutive 

 gene pairs is relatively rare, so that linkage 

 between such genes is almost complete. But 

 between more distantly localized gene pairs 

 cross overs are frequent, although, if the dis- 

 tance is too great, the cross-over values are 

 difficult to analyze — because double (Fig. 26- 

 18), or even triple cross overs may occur. 



The mathematical relation displayed by 

 the cross-over values of the yellow to ruby 



presynaptic CXmXXXXXOOOOBCCCCCaXXDO 



CHROMOSOMES " ~ ~ ~ 



SYNAPSIS 



POSTSYNAPTIC 

 CHROMOSOMES 



MMMMOMMM a 



••©••cooaxooeaxoca 



Fig. 26-18. Double crossing 

 over. If cross over occurs at the 

 points shown in this diagram, the 

 genes A and I, and also a and /, 

 will occur In their original com- 

 binations in the postsynaptic 

 chromosomes; but the cross over 

 will be evidenced by the inter- 

 change of the genes F and f at 

 an intermediate locus. 



