58 



45 plots produced constant black-kernelled plants. 



20 " showed a mixture of black and yellow kernelled plants. 



43 " showed a mixture of black, yellow and white kernelled plants. 



23 " showed a mixture of black and white kernelled plants. 



16 " produced constant yellow-kernelled plants. 



23 " showed a mixture of yellow and white kernelled plants. 



15 " produced constant white-kernelled plants. 



185 



The crossing in question was therefore clearly enough a dihybrid one, 

 since some plots contained only black and white-kernelled forms and others 

 only black and yellow. 



In the light of the Theory of Presence and Absence this strange 

 phenomenon seems easy of explanation. Instead of Black and Yellow form- 

 ing a single character-pair each of these acts independently of the other, 

 Black with the absence of black forming one pair and Yellow with the absence 

 of yellow forming the second pair. This crossing may be illustrated as 

 follows : 



B (black) 



X 



y (absence of yellow) Y (yellow) 



"On this theory/' says Punnett (55 p. 35) "the dominant character of an 

 alternative pair owes its dominance to the presence of a factor, which is absent 

 in the recessive." 



The Black oat is therefore black owing to the fact that it possesses a 

 factor for " blackness " which is absent in the recessive. Instead of the gamete 

 always carrying a definite" factor for either dominance or recessiveness it may 

 be regarded as either possessing or not possessing one of the factors of an 

 alternative pair; in other words the factor is either Present or Absent. This 

 conception will become clearer if we follow its application in detail to the 

 case of the above crossing. In this case the presence of each of the two 

 factors Black (B) and Yellow (Y) is alternative to its respective absence. 

 The Black-grained oat contains a factor for Black but not a factor for yellow, 

 while in a similar manner the Yellow-grained oat contains the factor for 

 yellow but not that for Black. In the above scheme the absence of Black 

 and Yellow has been indicated by a small " b " and " y " respectively for the 

 sake of convenience. 



As already indicated (See page 54) when two character pairs are in- 

 volved in a crossing as in the above case, there may arise in the hybrids four 

 kinds of egg-cells and four kinds of pollen cells. Either the egg-cells or pollen 

 cells may be represented as follows : BY, By, bY, by. If the four different 

 kinds of egg-cells unite in all possible ways with the four different kinds of 

 pollen cells involved in the above crossing sixteen different combinations 

 are possible. These may be represented symbolically as follow: 



