436 THE AMERICAN NATURALIST [Vol. XLTI 



individuals belonging to the theoretical F 2 ratio consists 

 of the following eight types : 



1 PBMPBM 



2 PBMPBm 

 2 PBMPbM 

 2 PBMpBM 

 4 PBMPbm 

 4 PBMpBm 

 4 PBMpbM 

 8 PBMpbm 



27 



There is only one basis on which a group of 9 indi- 

 viduals having a common gametic feature may be de- 

 rived from this group, namely, on the ground of homo- 

 zygosis with respect to any single allelomorph. Thus, 

 there are 9 homozygotes with respect to P (1 PBMPBM 

 + 2 PBMPBm + 2 PBMPbM - 4 PBMPbm), 9 homo- 

 zygotes with respect to B (1 PBMPBM + 2 PBMPBm + 

 2 PBMpBM + 4 PBMpBm), and 9 homozygotes with re- 

 spect to AT (1 PBMPBM + 2 PBMPbM + 2 PBMpBM 

 + 4 PBMpbM), and the assumption that any one of these 

 groups will give self-colored beans will answer the re- 

 quirements of the empirical F 2 ratio, 18:18:6:6:16. 



The only way in which it is possible to decide which 

 of these three possible groups of 9 homozygotes is respon- 

 sible for the equality of the mottled and self-colored 

 types is to test their applicability to the other genera- 

 tions, since they all fit equally well the ratio found in the 

 second generation. If homozygotes with respect to P 

 hide the presence of M, it will be impossible to find an 

 individual with mottled seeds which will not give a 

 progeny, one fourth of which will be white-seeded; but of 

 the F 3 families already examined, a number have been 

 found which, while continuing to give mottled and self- 

 colored beans in the ratio 1 : 1, have failed to produce any 

 whites. If the homozygotes with respect to B are re- 

 sponsible for the latency of M, some brown or In-own 



