NEO-MENDELISM IN PLANTS 415 
hypothesis do not use the form of notation thus far used in explaining 
Mendelian inheritance. Assume that T is used to express the deter- 
miner for tallness, its same letter (¢) is used to express the absence. 
For example, instead of using D for dwarfness, ¢ is used for “lack of 
tallness” (Fig. 77). It is a matter of convenience to have a symbol 
to represent the recessive, the absence of something that is present in 
another individual. 
In summary, the essentia] difference between the presence and 
absence hypothesis and that of dominant and recessive is that in 
the former case the recessive determiner has no existence at all, 
while in the latter case it exists, but is in a latent condition when 
associated with the dominant. 
BOC), 
Tal) Parent 
Dwarf Parent O/\O 
Fic. 77.—Diagram showing how presence and absence scheme is actually 
used, with small letter representing ‘‘absence.” (From Coulter and Coulter.) 
II. BLENDS 
This type of inheritance when first discovered was thought to be 
in direct conflict with Mendel’s law. It is a case in which dominance 
seems to fail, for the two alternative characters both express them- 
selves and the result is an average between them. It is easy to explain 
this situation in accordance with the presence and absence hypothesis 
without any violation of Mendel’s law. 
The classic example of blending inheritance was presented by 
Correns in breeding work upon Mirabilis Jalapa, the common four- 
o’clock. Correns crossed red and white varieties, and all the hybrid 
progeny had rose pink flowers. This was a color blend, distinctly 
intermediate between the colors of the two parents. The F, genera- 
tion, therefore, seemed to contradict: Mendel’s law in that one color 
character was not completely dominant over the other. The real situa- 
tion, however, appeared in the F, generation obtained by inbreeding 
