ALLELOMORPHIC RELATIONSHIPS IN MEN DELI SM 147 



Intermediate Expression in the Hybrid. From those cases in which 

 dominance is nearly or quite complete we may next pass to those in which 

 the character expression of Aa is intermediate to that of AA and aa. 

 There are numberless instances of this kind, and they are of interest 

 because the heterozygous class may be distinguished in F 2 , so that the 

 typical ratio obtained is lA:2Aa: la, instead of 3A:la as in cases where 

 dominance occurs. 



For a concrete example we may turn to Baur's case in the snap- 

 dragon. Baur and Miss Wheldale have independently conducted very 

 extensive investigations of Mendelian inheritance in Antirrhinum. 

 For most cases one member of a pair of contrasted characters is dominant, 

 but when ivory is crossed with red the FI is intermediate in color, it is 

 pale red or pink. When F z is grown it is found to consist of 1 red: 2 

 pink: 1 ivory. In one case among 97 plants, Baur obtained 22 red, 



RR 

 Red 



rr 



Ivory 



Rr 

 Pink 



1 RR 2 Rr Irr 



Red Pink Ivory 



I . I 



RR 1 RR 2Rr 1 rr rr 



Red Red Pink Ivory Ivory 



FIG. 67. Results of crossing snapdragons with red and ivory colored flowers. 



52 pink, and 23 ivory, a satisfactory agreement with Mendelian ex- 

 pectations. The actual proof for this case comes out in growing F 3 . 

 When this is done it is found that the red plants and the ivory plants 

 give progeny which are entirely red and ivory, respectively. The pink 

 plants on the other hand are all heterozygous and they give in F 3 and 

 in all succeeding generations plants in the proportion of 1 red : 2 pink : 1 

 ivory. The case is very evidently one in which a single factor difference 

 is concerned. If the factor responsible for the production of red in 

 Antirrhinum be designated by R, then we may designate its allelomorph 

 present in the ivory race by r. The case then works out according to 

 the diagram in Fig. 67. 



In the Four o'clock, Mirdbilis jalapa, it appears to be the rule that 

 heterozygous plants present visible differences from plants homozygous 

 for color factors. For this reason in breeding experiments this plant 

 gives a rather remarkable diversity of colors with relatively few factors 

 involved. Thus we may start with the primary assumption that in one 

 series of colors we have involved two pairs of factors as follows: 



Y = factor for yellow colored sap. 



R = factor which turns yellow sap red. 



