42 CELL HEREDITY 



str-d states so that the relation is really rectangular because two, not 

 one, mutable units underlie the phenotypes: 



Furthermore, when there are only two alternative phenotypes such as 

 his" and his^, we cannot so readily be sure that only one mutable unit 

 is involved, for the relation may really be: 



{his'^) Phenotype 



hisi^ his-i^ Genotype 



{his-) ^ ^ {his-) 



his{~ his-2^ his{^ his2~ 



\ /■ 



his I his2 



{hi.n 



It is clear then that, as usual, it is more difficult to show that two 

 things are identical, than that they are different. By a more careful 

 examination of the phenotypes, it is sometimes possible to discover small 

 differences between, say, the his ^~ his2^, his ^^ his^ and his ^~ his2~ 

 states. Or it may be shown that the his ^~ his., state exists by its 

 inability to revert directly to the his^ condition; it can revert only 

 through another his state in two mutaticMial steps. Furthermore, it is 

 sometimes possible to separate two otherwise indistinguishable mutable 

 units by recombination similar to that which took place in linked 

 transformation. ) 



Mutational as well as recombination analysis allows the identification 

 of so-called suppressor (sup) genes. These cause phenotypes resembling 

 those due to the wild-type state of another mutable unit. Thus what 

 appears to be a back mutation, e.g., his^^his'^, may in reality be a 

 genotypic change in a suppressor gene, e.g., his^ sup~ ^his~ siip'^. We 

 therefore define the mutable unit as the smallest part of the genetic 



