HEREDITY 



four types of germ cells AB, Ab, aB, and ab were formed 

 in equal quantities. If one multiplies egg cells AB -\- Ab -\- 

 aB + ab by sperm cells AB + Ab + aB + ab, having 

 regard for the phenomenon of dominance, plants duplex in 

 genetic constitution are obtained in the 9:3:3:1 ratio. This 

 was the situation actually found. 



Numerous experiments modeled after those of Mendel 

 were carried out during the early years of the twentieth 

 century; and gradually more and more complex cases of 

 inheritance yielded to factorial analysis. All possible 

 modifications of the fundamental di-hybrid ratio (also 

 tri-hybrid and higher ratios) of 9:3:3:1 were discovered. 

 These modifications were due to the fact that certain types 

 of genes produced results which made two or more of the 

 members of this ratio resemble each other superficially, 

 though their breeding powers were quite different. Thus 

 hybrids produced populations which could be divided into 

 distinct groups in such ratios as 13:3, 9:7, and 9:3:4; yet 

 continued breeding tests proved that each was simply a 

 modified 9:3:3:1 ratio. 



Perhaps the most important result of this early work was 

 the replacement of the old notion that like produces like by 

 the idea that each gene produces genes like itself. Exact analysis 

 of pedigree culture data had shown that individuals which 

 could not be distinguished from each other by any ordinary 

 test produced progenies which did not resemble each 

 other. Part of this interesting situation was cleared up by 

 the recognition of the important role played by dominance, 

 since AABB individuals would look like AaBB, AABb, and 

 AaBb individuals, though each type would behave differ- 

 ently in reproduction. But this was not the whole story. 

 It was found that genes AA could be the determinative 

 factors in the production of characters that would hide the 

 characters produced by genes BB. In such cases individuals 

 containing the genes AA (or Aa) would be alike in appear- 



[167] 



