INHERITANCE 311 



Mendel's great service lay in a long series of carefuJ 

 hybridizing experiments, from which the principle was 

 deduced that whatever the appearance of the hybrid, it pro- 

 duces germ cells like those of its parents in approximately 

 equal numbers, and the character of its own offspring will be 

 determined by the way in which these germ cells are paired 

 in fertilization. Suppose, for illustration, that D and R 

 of the accompanying diagram (fig. 181) represent the two 

 parents, which differ in one character only, that of color. 

 D is black and R is w^hite. Suppose also black to be the 

 dominant and white the recessive color. Then the offspring 

 in the first generation (F J will all be black. But if they be 

 bred together, their offspring will be both black and white 

 in the proportion of three black to one white. Then, if the 



whites be again bred together, all 



©^-^ their oft'spring will be white. The 

 ( ^ ) white character which disappeared 

 in the first hybrids, was obviously 

 still present, and has been sorted 

 out again. This relation between 

 characters in the germ cells has 



©^!^ f^~^ been aptly compared to the putting 

 ^g/^ \^__y together in pairs of pieces of glass 



of two kinds, one transparent, the 



^'L^\r^S^'''E:''^e Other opaque; when placed 



tfsfwf f^^Tf^^ £ together only the opaque one is visi- 



b?Ms'?esp4tfvdr''°'' ''^' ble,but when again separated, both 



are again apparent. 



If, as Mendel supposed, the germ cells possessing the two 

 characters separately, are present in equal numbers in the 

 reproductive organs of the hybrids and are combined in pairs 

 according to the law of chance, there are but four possible 

 combinations of them, giving three classes, as indicated 

 on succeeding page: 



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