J] and its Consequences 1 1 



generation are in exact agreement with this suggestion : 

 for the R plants give R only ; and of the D plants one- 

 third give D only, while two-thirds give the same mixture, 

 $D : iR, which was produced by F^ (Fig. 2. I). 



The descent may be represented diagrammatically 

 thus : 



Parents Tall ( TT) x Short (#) 



F! Tall (TV) 



F 2 TalT "TalT "faTT "lihort 



TT Tt tT tt 



pure tah \ pure short 



F S TT tt tr u fr^n~7r~it 



Now since the fertilised ovum or zygote, formed by the 

 original cross, was made by the union of two germ-cells or 

 gametes bearing respectively tallness and dwarfness, both 

 these elements entered into the composition of the original 

 FI zygote ; but if the germ-cells which that zygote eventually 

 forms are bearers of either tallness or dwarfness, there 

 must at some stage in the process of germ-formation be a 

 separation of the two characters, or rather of the ultimate 

 factors which cause those characters to be developed in the 

 plants. This phenomenon, the dissociation of characters 

 from each other in the course of the formation of the 

 germs, we speak of as segregation, and the characters which 

 segregate from each other are described as allelomorphic, 

 i.e. alternative to each other in the constitution of the 

 gametes (Fig. 2). 



That this is the true account was proved by further 

 experiments which Mendel made by crossing the F 1 with 

 pure dominants and with pure recessives. For DR x DD 

 gave an offspring all dominant in appearance, though in 

 reality consisting of both DR plants and DD plants, on an 

 average in equal numbers. On the other hand DR x RR 

 gives an equal number of dominants and recessives, of 

 which the dominants are all DR plants, and the recessives 

 are all pure recessives. These various experiments illus- 

 trate the composition of the four simple types of Mendelian 

 families, which may be set out thus : 



