.1 NEW MENDELIAN RATIO 



435 



bean as a simple Mendelian unit, while he assumed that 

 a mottled factor was carried as a "cryptoinerc" hv the 

 pigmented bean and that the white bean acts simply as 

 a releasing agent or activator which allows or compels 

 the latent mottling to become apparent. 



The ratio 18:18:6:6:16 must have at first a very un- 

 familiar look to the student of genetics. It was not ex- 



The census of my second generation was completed 

 shortly after the appearance of De Vries's 2 interesting 

 account of ' 'Twin hybrids" in Oenothera, and the sug- 

 gestion lay at hand that this ratio presented by Phaseolus 

 might be a case of twin di-hybrids, the first and second 

 terms of the ratio, as also the third and fourth term.-, 

 being in each case different phases or aspects of a single 

 unit, which might be expressed thus 9A:9Y :3B:38 :8W. 

 While such an hypothesis would fit the conditions pre- 

 sented by the F 2 ,' it was seen very soon that it does not 



mottled F„ nor with the splitting phenomena 'of F,! a 

 portion of which has been already examined. A consid- 

 eration of the F, and F 3 shows that there are three dis- 

 tinct units involved, as was stated in my earlier papers, 

 namely-a pigment factor, P, a blackener, B, and a mot- 

 tled pattern, M. 



If all of these characters behaved according to the 

 simple Mendelian method, the ratio would be that pre- 

 viously predicted, and out of every 64 individuals, on an 

 average, 27 would have purple mottled seeds, and 9 black. 

 In order that the number of individuals having purple 

 mottled seeds shall be equal to the number having black 

 seeds, it is necessary that of the 27 that should on theo- 

 retical grounds be purple mottled, 9 must show no purple 

 mottling but must be black, though it contains the domi- 

 nant mottle factor, M. This group of 27 purple mottled 



