44 MENDELISM CHAP. 



of all these three sorts of reds was demonstrated by 

 experiment, and the proportions in which they were 

 met with tallied with the theoretical explanation. 



The theory was further tested by an examination 

 into the properties of the various F 2 whites which 

 come from a coloured plant that has itself been 

 produced by the mating of two whites. As 

 Fig. 7 shows, these are, in respect of their constitu- 

 tion, of five different kinds, viz. AAbb, Aabb, aaBB, 

 aaBb, and aabb. Since none of them produce any- 

 thing but whites on self-fertilisation it was found 

 necessary to test their properties in another way, 

 and the method adopted was that of crossing them 

 together. It is obvious that when this is done we 

 should expect different results in different cases. 

 Thus the cross between two whites of the constitution 

 AAbb and aaBB should give nothing but coloured 

 plants ; for these two whites are of the same con- 

 stitution as the original two whites from which the 

 experiment started. On the other hand, the cross 

 between a white of the constitution aabb and any 

 other white can never give anything but whites. 

 For no white contains both A and B, or it would 

 not be white, and a plant of the constitution aabb 

 cannot supply the complementary factor necessary for 

 the production of colour. Again, two whites of the 

 constitution Adbb and aaBb when crossed should 

 give both coloured and white flowers, the latter 

 being three times as numerous as the former. 

 Without going into further detail it may be stated 

 that the results of a long series of crosses between 

 the various F 2 whites accorded closely with the 

 theoretical explanation. 



