138 THE FIRST PRINCIPLES OF HEREDITY 



essential, each white bringing in a separate colour factor. 

 If we denote the two colour-producing factors by A and B 

 (both being dominant), and the absence of the colour- 

 producing factors by a and b (being recessive), then we 

 get the following scheme : 



White (A Variety) x White (B Variety). P 



Two allelomorphic pairs : 



i. Colour factor A 

 Absence of Colour a 



2. Colour factor B 

 Absence of Colour b 



Colour factor A^j 



+ }= Purple 



Colour factor B 



9 AB + 3 A& + 3 aB + i ab 

 9 Purple 7 White F 2 



FIG. 60. SWEET-PEA. (After Bateson.) 



AB, containing both colour factors, is purple ; but A&, 

 aB, and ab are all white, because they either contain only 

 one colour factor (A or B) or none. 



Here once more, the proportion 9 : 7 only masks 

 the Mendelian proportion of 9 + 3 + 3 + 1, the latter three 

 types being identical in appearance, though in reality of 

 a different constitution. 



The two cases last discussed throw incidentally some 

 light on the phenomenon of Reversion. The white colour 

 of the sweet-pea is a new character, which, on intercrossing, 

 reverts to the ancestral purple flower. Similarly, the cross 

 of a black and albino rabbit yields the wild grey form. 

 Evidently reversion in both cases is due " to the meeting 

 of factors belonging to distinct allelomorphic pairs," 

 which in the new forms have become separated. On 

 crossing the new forms, these necessary factors once more 

 reunite, producing the old racial type. 



