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THE PRINCIPLES OF HEREDITY 



of this it is possible to determine the positions of genes on the chromosomes 

 and the relative distance between different genes on the chromosomes of 

 those forms of life which have been studied thoroughly from a genetic 

 standpoint. 



Crossing over is of great value in the establishment of desirable 

 genes through the rigorous process of natural selection. If all of the 

 genes lying on one chromosome were inseparably linked, then selection 

 would have to be on the basis of entire chromosomes. Every chromo- 

 some certainly will contain some genes which are good and some which 



Photo by Winchester 

 Fig. 32.7. Crossing over as seen in living cell from grasshopper testes. This highly 

 magnified photomicrograph shows a pair of chromosomes in the prophase of the first 

 division of meiosis. The two chromatids which make up each chromosome can be 

 seen. Note that crossing over has taken place between opposite chromatids at four 



places. 



are not good in their effects. Through crossing over the desirable genes 

 may be separated from the undesirable, and through natural selection 

 the good may become widely established while the undesirable genes 

 are reduced in numbers. Thus, crossing over allows natural selection 

 to take place on the basis of individual genes rather than on the basis of 

 the total influence of entire blocks of genes which lie on the same 

 chromosomes. 



Multiple Genes and Quantitative Characteristics 



Common observation readily reveals the fact that all of the inherited 

 characteristics of living things do not follow the pattern of simple 



