MENDELISM 



to the building up of a type which is a homozygote in 

 respect of both the necessary factors. 



This may be made clearer by a definite illustration. 



A pea-plant characterized by the presence of a greyish 

 or brownish testa to its seeds (grey) was crossed with 

 a plant having nearly colourless test as (white). The 

 testas of the F x plants were marked with bright purple 

 dots on a grey ground (purple). These hybrid plants 

 were self-pollinated, and in F 2 the three types appeared 

 in the following proportions : 9 puiple, 3 grey, 4 white. 

 What is the meaning of this ratio ? In order to 

 complete the ordinary expectation for a simple Men- 

 delian case in which two pairs of allelomorphs are 

 concerned (di-hybridism) we must write down the 

 following expression : 



I purple \ /no purple \ . / purple \ . , /no purple \ 

 I grey Jl grey / ' J \ no grey / ' \ no grey / 



But it would seem that the purple character cannot 

 appear when the grey colour, or some factor con- 

 stantly associated with this colour, is absent, as is 

 the case in the original white parent from which the 

 factor for purple spots was derived. Consequently, the 



three J P P I plants are indistinguishable from the 

 [no greyj 



{no purple 1 , , ., 



r r I plants or whites, and we thus arrive at 

 no grey J 



the result which was described as being the one actually 

 obtained namely, 9 purple : 3 grey : 4 white. 



In other respects this example is precisely like the 

 case of two pairs of allelomorphs described on p. 176. 



