THE MUTABLE UNIT OF HEREDITY 41 



Consider for a moment what would be expected of two mutable units, 

 each in two alternative states. We can see that there are four possible 

 combinations: 



a+6+ . Wild type 



a~b^ a^b~ Single-step mutants 



a b Two-step mutant 



Because each of the units mutates independently, two mutational 

 steps are required for the genetic material to pass from one condition 

 to the opposite (e.g., from a~b~ to a^b'^). If only one mutable unit is 

 involved and there are more than two states, each may mutate to 

 another in one mutational step: 



a' . 



a 



Now in practice, of course, we observe only phenotypes. If two 

 unrelated phenotypes, such as growth on streptomycin and the ability 

 to make histidine, are involved, there are four possible phenotypes. If 

 among them we find such a relation as 



str-s his^ . 



. ^ % 



str-r his^ str-s his 



str-r his 



we can be sure that the pattern is caused by the mutation of two 

 mutable units, not one. Sometimes what may seem to be three mutually 

 exclusive phenotypes are found in the following pattern: 



str-s 



// V 



str-r ^ str-d 



It could be that one mutable unit is here being observed in three of its 

 states. On the other hand, it may be that there are two indistinguishable 



