90 CELL HEREDITY 



observe a segregation ratio. But e\cii this is difficult to do because 

 human matings, fortunately, cannot be made at the will of an experi- 

 menter. .Vs Beadle has written, the choice of a man s mate is not often 

 based on a desire to contribute to the knowledge of heredity and, 

 furthermore, it is inconvenient to subject him to a complete chemical 

 analysis. As a consequence, human heredity must be studied by the ex- 

 amination of family trees and by other indirect methods. With 'lower' 

 plants and animals, however, sufficiently large populations of offspring 

 can be collected from identical matings. and ratios such as the type 

 9:3:3:1 can be tested directly. Three-factor crosses yield, of course, 

 more complicated ratios. These ratios can be worked out mechanically 

 by the same methods we have employed thus far. They will be 

 described in any textbook of classical genetics. The more complex 

 the ratios become, the larger the number of siblings that must be col- 

 lected to establish them. 



It should be evident, however, that complex as the combinations 

 resulting from the assortment of many genes may be, the behavior of 

 the individual genes is simple and is only a consequence of the chromo- 

 some mechanics of meiosis and fertilization. We shall next consider 

 some of the more specific correlations between genes and chromosomes. 



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