No. 409 J A NEW MENDELIAN RATIO 435 



bean as a simple Mendelian unit, while he assumed that 

 a mottled factor was carried as a ' ' cryptomere " by 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- 

 plained by Tschermak, but was separated by him into two 

 groups of 9 : 3 : 4, wherein the interrelations of the several 

 terms need no explanation. 



The census of my second generation was completed 

 shortly after the appearance of De Vries's 2 interesting 

 account of "Twin hybrids " in (Enothera, 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 terms, 

 being in each case different phases or aspects of a single 

 unit, which might be expressed thus 9A:9Y :3B :39 :8W. 

 While such an hypothesis would fit the conditions pre- 

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

 harmonize with the occurrence of a uniformly purple 

 mottled F 1? nor with the splitting phenomena of F 3 , 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 



2 Do Vries, H. On Twin Hybrids. Bot. Gaz., 44, pp. 401-407, D 1907. 



