204 MENDELISM 
or brownish testa to its seeds (grey) was crossed with 
a plant having nearly colourless testas (white). The 
testas of the F, plants were marked with bright purple 
dots on a grey ground (purple). These hybrid plants 
were self-pollinated, and in F, the three types appeared 
in the following proportions : 9 purple, 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 : 
ope} ss (gm) ss (Bom, } + (ae) 
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 { aga plants are indistinguishable from the 
no grey 
no purple 
{ no grey 
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. 187. 
We may write A for presence of grey pigment, a for 
absence of grey pigment, B for presence of purple, and 
6 for its absence. Then the original cross was of the 
form Ab xaB, from which AaBod resulted in F,. And 
} plants or whites, and we thus arrive at 
