Heredity 181 



and smooth wheat the hybrids usually show a slight tend- 

 ency to be bearded. Likewise, the cross between horned 

 and polled cattle may have scars (hornlessness is dominant) . 

 It frequently happens that instead of either of two opposite 

 characters being dominant, we get a form intermediate 

 between the two parent forms. Thus, in the cross be- 

 tween long-headed wheat and the short-headed club 

 wheats of the Pacific Coast, the hybrids have heads of 

 intermediate length, though they are much more like 

 club wheat than they are like the ordinary kinds, so that 

 the club character is at least partially dominant. In cer- 

 tain crosses between red-flowered and white-flowered 

 ornamental plants the hybrids are pink." 



Presence-and-absence hypothesis. The phenomena of 

 mendelian inheritance may be explained in one of two 

 ways : first, the presence of a definite substance in the 

 germ cells of both parents representing each unit-character 

 in the allelomorphic pair, and, second, the " presence-and- 

 absence" hypothesis. The latter assumes that what 

 appears to be a pair of characters is really the presence 

 and absence of a single character. 



Examples of mendelian inheritance due to the presence- 

 and-absence of a single unit. Red flowers may be 

 due to the presence of red, and white flowers to its 

 absence. 



The wrinkled pea owes its character to the absence of 

 something which the round pea possesses. Darbishire 

 has found in the round pea that all of the sugar has been 

 converted into starch, while only a part of it has been 

 thus converted in the wrinkled pea and the wrinkling is 

 primarily due to the escape of the water from the solu- 



