MendeVs Law 



SI 



absence hypothesis do not use the fomi of notation thus 

 far used in explaining Mendehan inheritance. Assume 

 that T is used to express the detenniner for tallness, the 

 same letter (/) is used to express its absence. P^or 

 example, instead of using D for dwarfness, / is used for 

 ''lack of tallness" (fig. 7). It is a matter of conven- 

 ience to have a symbol to re])resent the recessive, the 

 absence of something that is present in another indi- 

 vidual. 



Dwarf Parent 



Cameti 



es 



■ Fig. 7. — Diagram showing how presence and absence scheme is 

 actually used, with small letter representing "absence." 



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. 



As a matter of fact, some of the later investigations have 

 revealed cases that can hardly be accounted for by the presence 

 and absence hypothesis. In spite of these recogni/.ed e.\ceptions, 

 however, the scheme of notation employed under the presence 

 and absence hypothesis has proved so convenient that it is 

 almost universally employed. 



