THE FACTOR HYPOTHESIS AS APPLIED TO PLANTS 



255 



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 (t) is used to express the absence. 

 For example, instead of using D for dwarfness, / is used for "lack of 

 tallness 1 ' (Fig. 59). 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 essential 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. 



Dwarf Parent 



Fig. 59. — 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 z 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 2 generation obtained by inbreeding 



