384 MCJDliRX THEORIES OF HEREDITY. 



calculated on the assumption that instead of the gametes segre- 

 gating in the proportion of i.A.B.: i.A.b. : i.a.B.: i.a.b., we have 

 7.A.B. : I.A.b.: La.!).: /.a.b., agree very closely with observation. 

 (Other examples give numbers approximating to a 15: i : i : 15 

 basis.) An example of this is found in certain sweet-peas. In 

 this case two of the allelomorphic pairs involved are purple (D) 

 and red colours, and long (D) and round pollen-grains. Here 

 we find that in F.2, considering the two pairs separately, we 

 have the normal ratios of 3 purple: i red. and 3 long: T round, 

 but if we examine the ])ollen of flic f^itrplc only we find a much 

 larger proportion of long grains, virs.. 12: 1, while in the reds the 

 ])roportion of longs is only i : 3.2. Coml)ining the two, we find 

 that the ])ro])ortions in F.2 are approximately as follows (actual 

 numbers in brackets) : — 



Purple flower and long ])ollen 177 ( 1528) 1 I 9 



Purple flower and round pollen is (106) . 13 



Red flower and long pollen 15 (117) '"stead ot j ^ 



Red flower and round i)ollen 49 ( 381 ) ) ' i 



In other cases of Mendelian inheritance we meet with the 

 remarkable phenomenon usually called " reversion," or some- 

 times " latency of characters." This may be best explained by 

 an example, and two varieties of sweet-pea wdll again be used. 

 These are two which difl^er in the colour of the seed-coat, one 

 being grey, and the other colourless (or white) F.i showed a 

 testa with purple dots on a grey ground ( pnrPlc) F.2 showed 

 a proportion of 9 purple: 3 grey: 4 white. The explanation is 

 that there are two pairs of allelomorphs — (t) purple and its 

 absence; (2) grey and its absence. But ]nirple is apparently 

 unable to appear without grey, hence : — 



[ purple ] (no purple) I purple | j no purple] 



9 : 3 - [:3 \ -I 



I grey ) ( grey i (no grey) ( no i^rey j 



appears as above, since ' ' I equals white. It is evident 

 ' ' I no grey ) 



that the original "white" parent was reallv P^'^P ^ [, 



' -^ ( no grey 1 



A second example where two latent characters give rise to a 



new character is furnished by the sweet-pea known as " Emily 



Plenderson." This is found to exi.st in two forms only dift'ering 



in the shape of the pollen-grains, which are "long" (D) or 



" round." The flowers are pure white in both cases. F.i lias 



flowers with a purple standard and blue wings (the type known 



as "Purple invincible"). F.2 (from self-pollinated F.i) shows 



the following proi)ortions : — 



" Purple invincibles " 81) ,3 



" Picotee " 27 i °'' 1 1 



" Painted lady " 27; f 3 



" 1 mged white 9 > (1 



"White" 112 - - - 



