ALLELOMORPHIC RELATIONSHIPS 1N MENDELISM 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 2, so that the 
typical ratio obtained is 14:2Aa:1la, instead of 34:1la 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 F; is intermediate in color, it is 
pale red or pink. When fF» 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 x TT 
Red Ivory 
Rr 
Pink 
1RR 2 Rr lrr 
Red Pink Ivory 
| SSS | 
RR 1RR 2Rr irr tT 
Red Red Pink Ivory Ivory 
Fic. 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 F3. 
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; 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, Mirabilis 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. 
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