LATENT CHARACTERS 191 



We may write A for presence of grey pigment, a for 

 absence of grey pigment, B for presence of purple, and 

 b for its absence. Then the original cross was of the 

 form AbxaB, from which AaBb resulted in F!. And 

 the visible characters of the types which appeared 

 in F 2 would be represented by gAB + ^Ab + (^aB f 

 lab). . On referring to the account given on p. 176 it 

 will be seen that one in nine of the purple plants is of 

 the constitution ABAB, and may be expected to 

 breed true. 



A precisely similar result may be obtained in F 2 in 

 cases where there is no reversion in F r In the 

 following example a white pea, which did not contain 

 the latent purple factor, was crossed with a ' maple- 

 seeded ' pea. The characteristic feature of maple is a 

 marbling of brown spots on a grey ground colour. In 

 F! the marbling was dominant, and the seeds resembled 

 the maple parent. 



In F 2 there appeared 9 maple : 3 grey : 4 white 

 i.e., the same ratio as in the previous case, this time 

 without reversion. This ratio is brought about by the 

 simple combinations of two pairs of allelomorphs A-a, 

 and C-c, C being unable to manifest itself unless A is 

 present in the same zygote. As a matter of fact, in 

 this particular case C does sometimes just manage 

 to appear in the absence of A, the result being a 

 white seed with a sort of faint ' ghost J of a maple 

 marking. 



When a strain bearing both maple marking and 

 purple spots is crossed with a white in which neither 

 of these factors is latent, we can easily calculate the 



