vj MENDELIAN HEREDITY 57 



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 F* will be explained immediately.) 



Original parents T x t 



I 

 fi Tt 



F* TT 2Tt 



F 3 TT TT 2Tt tt tt 



It is thus clear that among the offspring of the F l 

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

 tall 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 F parent. 



The explanation offered by Mendel of these facts 

 was as follows. The original tall plant produces 

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

 plant produces gametes bearing shortness (absence 

 of tallness). The F l (hybrid) thus contains both 

 conditions ; its cells, resulting from the union of two 

 gametes, may be regarded as double structures, con- 



