vj MENDELIAN HEREDITY 67 



fertilised, while others again give a mixture of tails 

 and shorts. The whole result may be clearer in 

 symbolic form. If T stands for the tall factor, t for 

 its absence (shortness), the following results appear. 

 (The "2Tt' in Fi will be explained immediately.) 



It is thus clear that among the offspring of the Fi 

 (hybrid) generation, some (tt) have eliminated the 

 taU factor altogether and show no difference from 

 their short ancestors ; others (TT) have nothing but 

 the tall factor and thus breed true to tallness ; and 

 a third group, which Mendel found was twice as 

 numerous as either of the others (therefore marked 

 2Tt), proved, by giving mixed offspring when selfed, 

 that it is hybrid like its jPi parent. 



The explanation offered by Mendel of these facts 

 was as follows. The original tall plant produces 

 germ-cells ('gametes') bearing taUness ; the short 

 plant produces gametes bearing shortness (absence 

 of tallness). The Fi (hybrid) thus contains both 

 conditions ; its cells, resulting from the union of two 

 gametes, may be regarded as double structures, con- 



