2 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



ment in the germ-plasm there is a normal partner of that 

 element which we call its allelomorph. We can not study the in- 

 heritance of one member of such a pair of genes without at the 

 same time studying the other. Hence whatever we learn about 

 those hereditary elements that stand for defects, we learn just as 

 much about the behavior of the normal partners of those elements. 

 In a word, heredity is not confined to a study of the shuffling of 

 those genes that produce abnormal forms, but is equally concerned 

 with what is going on when normal genes are redistributed. This 

 method of pitting one gene against the other furnishes the 

 onlv kind of information relating to heredity about which we have 

 precise knowledge. 



In man and in domesticated animals we find that individuals 

 appear occasionally that are defective in one or another respect. 

 Some of the defects are inherited. Rarely a new one appears that 

 has not been seen before. But the majority of them are reap- 

 pearances of characters that have been carried under the surface 

 as recessive genes in the germ-plasm. Today we recognize that 

 each of these modifications, if recessive, has first arisen as a mu- 

 tational change in a single gene before it appeared on the surface 

 as a character by the coming together of two such genes. Men- 

 delism has furnished some information as to the way in which 

 these hidden genes may get dispersed in the race. An example 

 will serve to make this clear, Fig. i. 



If a fly with vestigial wings, a recessive character, is crossed 

 to a wild fly with long wings, all the offspring ( l ; ,'s ) will have 

 long wings. If these are bred to each other the offspring will be 

 of two kinds, like their grandparents, in the ratio of three long 

 winged to one vestigial fly. The extracted vestigials will breed 

 true to vestigial. The fact that the gene for vestigial has been 

 carried by long winged F a parents has not affected the gene in any 

 way, for the second generation of vestigials has wings as short 

 as those of their grandparents. 



I have brought forward this case not so much to illustrate 

 Menders law of segregation as to use the facts for another pur- 

 pose. 



