A. D. Darbishire. 
276 
to dominance might put it down as such without further enquiry. 
It is, however, nothing of the kind, as we shall shortly see. 
But what is the explanation ? The theory which naturally 
presented itself was that M and P are not allelomorphic to one 
another—but that each is the dominant member of a pair, the 
corresponding recessive member of which was the absence of that 
particular character. That is to say the two pairs are Maple and 
its absence ; and Purple spot and its absence. We may conveniently 
denote the absence of M by in, and the absence of P by p. If this 
theory is true the possession by the hybrid of the two characters is 
no more an instance of a blend, than is the fact that the result of 
crossing a yellow wrinkled Pea with a green round one is a yellow 
round hybrid which, just as in the case we are now considering, 
derives one character (yellowness) from one parent and the other 
(roundness) from the other. 
That this theory does approximate fairly closely to the truth is 
shown by the composition of F 2 . For if the two pairs of characters 
(M-m and P-p ) are inherited separately, and segregation occurs in 
the manner in which, as we have seen, the characters of the coty¬ 
ledons were inherited, we should expect F 2 to have the following 
average composition :— 
9 AB : 3 Ab : 3 aB lab 
in which A is the dominant, and a the recessive member of one 
pair, and B the dominant and b the recessive member of another 
pair. That is to say, in this particular case we should expect F 0 to 
have the following average composition :— 
9 MP : 3 Mp : 3 mP : 1 mp 
The experiment has not been carried out on a scale sufficiently 
large to justify the statement that these proportions obtain. But 
these four types have been recognized, and occur in proportions 
which do not differ widely from expectation. 
I well remember the excitement with which I opened the dry 
pods of the plants of this F 2 generation. Plants with seed-coats 
which bore both M and P were in the majority ; plants with M only, 
or P only, occurred in smaller quantities than MP, though in equal 
numbers. I had examined many plants before I found one bearing 
the mp character; and as I had not a very large number of plant?, 
and this character is only expected to occur once in every sixteen 
individuals I became afraid that I might not see one. But I did 
ultimately obtain two such plants. And well do I remember the 
interest with which I looked on these Peas, in which the two units 
