466 AN INTRODUCTION TO ZOOLOGY 



Plants bearing smooth seeds were crossed with those bearing 

 wrinkled seeds, and the results obtained can be expressed diagram- 

 matically. In the first filial generation (F. i), all the peas were 

 smooth, and the wrinkled character had disappeared. When 

 members of this generation were interbred, however, it was found 

 that in their offspring, the second filial generation (F. 2), wrinkled 



F.I. ..(S), 



/ /\ 

 * F. 2. S (S) (S) W 



seeds again made their appearance, and in the proportion of i 

 wrinkled to 3 smooth. It is interesting to recall the actual results 

 obtained by Mendel to see how closely they agree with the pro- 

 portions given : 258 plants yielded 8023 seeds, 6022 yellow and 

 2001 green ; their ratio, therefore, was 3'0i to i ; 253 plants yielded 

 7324 seeds, 5474 round and 1850 wrinkled ; their ratio was, therefore, 

 2*96 to i. Further investigation showed that the three plants 

 with smooth seeds did not all behave in the same way. One of them 

 was similar to the smooth-seeded plants with which we started, and 

 if inbred would never produce anything but smooth seeds, no matter 

 how many generations were tried : it was, as we say, '* pure." The 

 other two behaved similarly to that of the F. i generation, and of 

 their offspring one- third always produced wrinkled seeds, so that 

 they were impure. The true ratio then of the plants in the Fig. i 

 generation is i : 2 : i. For a character that we can examine in this 

 sort of way the term " unit character " is employed, and when we 

 find a pair of such characters that are mutually exclusive, they may 

 be termed " allelomorphs," as they were called by Bateson. Mendel, 

 in order to describe the masking that occurs in the F. i generation, 



D X R 

 F. i. 

 F. 2. D DR DR R 



proposed the term dominant for that character that appears in the 

 F. i generation, and recessive for the one that does not, so that we 

 can express the result in a generalised diagram. This result is 

 sometimes referred to as Mendel's Law of Dominance, although it is 

 not, strictly speaking, a law, since it is not of general application, 



