CHAPTER XIV 



THE THEORY OF REVERSION 



The Mendelian theory by which it is sought to 

 explain the phenomenon of reversion will be dealt with 

 in this chapter. By way of illustration, the result 

 of crossing a white-seeded with a grey-seeded pea, 

 and that of crossing a white -flowered with a pink- 

 flowered variety of pea will be taken (see Fig. 27 

 and Plate IV.). 



The two pairs of characters in the colour of the 

 seed-coat with which we are concerned here are 

 purple-spot (P) and absence of purple-spot (p), and 

 grey (G) and absence of grey (g). According to 

 the theory, the character P has this remarkable 

 property, that the zygote which contains it cannot 

 manifest the purple-spot unless the character G be 

 present also. The formulae for the gametes of the 

 white-coated and grey-coated pea crossed are, for 

 the grey-skinned, Gp, namely, presence of grey and 

 absence of purple-spot; and for the white -skinned, 

 gP, namely, absence of grey and presence of purple- 

 spot, which, however, cannot be manifested in the ab- 

 sence of the grey-coat. Suppose the white -coated were 

 used as the female parent, the union of the gametes 

 and its result would be represented as follows : — 



Gametes of white Gametes of grey 



seed-coated $ parent seed-coated ^ farent Zygote produced 



gP X Gp = GgPp. 



