MENDEL'S FIRST LAW 25 



That MendePs principles apply to animals was first 

 made out by Bateson and by Cuenot in 1902. Since then 

 many characters both in domesticated and in wild animals 

 and plants have been studied, and there can be no question 

 of the wide application of MendePs discovery. 



During the years immediately following the re-dis- 

 coveiy of MendePs principles (1900) much attention was 

 paid to the phenomena of dominance and recessiveness. 

 This was due, no doubt, to the striking fact that the hybrid 

 sometimes resembles only one parent in some particular 

 trait, whereas the older observations, where many charac- 

 ters were generally involved in the cross, seemed to have 

 shown that hybrids are intermediate in regard to their 

 parents. We now know, however, that although there are 

 cases in which the dominance is as complete as in those 

 described by Mendel, yet in a very large number of forms 

 the hybrid is intermediate between the parents, even 

 when only a single pair of characters is involved. A few 

 examples will serve to illustrate these relations. 



The common garden four o 'clock, Mirahilis jalapa, has 

 a white-flowered and a red-flowered variety (Fig. 2). 

 When crossed, the hybrid has a pink flower, which may be 

 said to be intermediate in color between white and red. 

 Here neither color can strictly be said to dominate. When 

 the hybrid (F^) is self-fertilized the offspring {F2) are 

 in the proportion of one white, to two pink, to one red- 

 flowered plant. The Fn reds and the F2 whites breed true ; 

 the pinks when self-fertilized give white, pink and red in 

 the proportion of 1 : 2 : 1. In a case of this kind the color 

 of the F2 plants reveals the nature of the three classes 

 present, so that it is not necessary to test them out, as was 

 the case in the F2 generations of MendePs peas, where 

 the Fo tails were found in this Avay to be of two sorts. 

 The F2 results w^ith the four o'clock also show that 

 the segregation of the genes is clean, for the F2 whites 

 never produce in subsequent generations anything 



