Weiss, Researches on Heredity in Plants. 



Each combination with a Y and R would give a form 

 characterised by the two dominant characters, and it will 

 be seen that they occur in nine combinations, while the 

 single dominant character Y occurs in three, and the 

 single dominant R also in three. The two recessive 



o 



characters without dominants only occur in one of the 

 sixteen combinations. Thus the ratios 9:3:3:1 is 

 accounted for. 



This numerical ratio of coupled characters has been 

 confirmed in several other instances by later experi- 

 menters both in the case of animals and of plants. 



In some instances what appear at first sight to be 

 unit characters have turned out to be pairs of allelomorphs, 

 showing the ratios 9:3:3: i in place of the 3 : i ratio. 

 This has been shown to be the case especially in cases of 

 heredity of colour, where certain unseen colour factors or 

 determiners come into play, such as are referred to above 

 in connection with the mating of two white varieties of 

 sweet peas. 



Again, differences in colour of flowers may be due to 

 very distinct causes. Thus in the Common Avens{Geu7/i 

 urbanuni^j the yellow colour of the petals is due to the 

 presence of yellow colour in the plastids of the petals. 

 In the Water Avens {Geuju rivale\on the other hand, the 



