34 THE QUANTITATIVE METHOD IN BIOLOGY 



But, as the first cross (between two subspecies) showed, when 

 D meets R in fertiUzation the resulting individual (FJ is in 

 appearance D (the property R being latent) ; therefore F2 

 appears as 3Z) : iR (the property R being concealed in DR and 

 RD). The results observed in the Fg generation are in exact 

 agreement with this suggestion, for the RR plants (in Fg) give 

 an offspring with the property R only, and of the plants in 

 which D is visible one-third {DD) give D only, while two-thirds 

 {DR + RD) give the same mixture (3/) + !^) which was pro- 

 duced by Fi. 



Now since the fertilized Fj ovum, formed by the original 

 cross, was made by the union [D $ >^R% or R$ xD^) oi two 

 germ-cells bearing respectively tallness and dwarf ness, both 

 these elements (possibilities) entered into the composition of 

 the generation F^ ; but if the germ-cells which that genera- 

 tion eventually forms are bearers of either tallness or dwarf ness, 

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

 separation of the two properties (possibilities). This pheno- 

 menon, the dissociation of properties (possibilities) from each 

 other in the course of the formation of the male and female 

 germs, we speak of as segregation. 



In the above-described experiment plants were used which 

 differed only in one essential property. MENDEL crossed 

 also subspecies of Pisum sativum which differed in two pro- 

 perties — for instance, a subspecies a with round seeds (property 

 D) and yellow "albumen'' (property d) crossed with a sub- 

 species b the seeds of which were wrinkled (property R) and 

 the "albumen" green (property r). The produced seeds Fj 

 appeared Dd like those of the parent a. In each pair of pro- 

 perties {D, R and d, r) there was thus a dominant (Z), d) and a 

 recessive {R, r). The cross-bred Fj bore seeds Fg by self- 

 fertilization. The Fg generation proved to consist (with 

 regard to the visible properties) of four kinds ^ of seeds — viz. 

 (a) Dd (round, yellow) 

 (p) Rd (wrinkled, yellow) 

 (y) Dr (round, green) 

 (S) Rr (wrinkled, green) 



^ In fact, nine combinations occur among the F2 plants. This may be cal- 

 culated by working out 



{D + Rfx{d + rf 

 The obtained terms are : 



(a) DH'^ + 2DRd^ + 2DHY + ^DRdr^ + 

 W) d^R^ + 2drR^ + 



(7) £)V2 + 2Z)i?>'2 + 



(5) J?V2 

 In each combination in which R meets D the property R is latent (concealed) ; 

 in the same way r is latent when meeting d. Therefore, as is easily seen, the 

 nine combinations are reduced to four groups, a, /3, 7 and d, with reference to 

 the visible properties. (See, on the use of formulas, §§ 104-106.) 



