THEORY OF REVERSION 223 



The three whites — and here we come to the really 

 interesting part of the sixteen-square Table illus- 

 trating the union of gametes of reversionary hybrids 

 — the three whites in squares 11, 12 and 15 correspond 

 to the green rounds, and bear the same relation to 

 the greys as the green rounds bore to the yellow 

 wrinkleds ; that is to say, they possess the recessive 

 factor for the character in respect of which the grey 

 was dominant, namely, absence of grey ; and they 

 possess the dominant factor for the character in 

 respect of which the grey was recessive, namely, 

 purple-spot, which, however, is not seen in the seed- 

 coat because this is white and not grey — the presence 

 of grey, as already explained, being a necessary 

 condition for the manifestation of the purple-spot. 

 The white in the corner (in square 16) corresponds to 

 the green wrinkleds, possessing, as it does, two reces- 

 sive characters ; and it differs from the other whites 

 in that it is a pure white, i.e. it does not possess 

 the factor for purple-spot as the three other 

 whites do. 



The parallelism between the Tables giving the 

 9:3:4 ratio and that giving the 9:3:3:1 is com- 

 plete in every particular. For instance, in the 

 former, as in the latter, the four zygotes homozygous 

 in respect of both their characters lie along a diagonal 

 passing across the Table from the top-left to the 

 bottom-right corner, whilst the four zygotes hetero- 

 zygous in both their characters (all purple-spotted 

 greys in this Table, as all yellow rounds in the other) 

 lie along the other diagonal. 



