DI-HYBRID CROSSES 189 
The three individuals of appearance aB include the 
following types : 
One pure, aBaB, breeding true to the (new) 
smooth white type. 
Two aBab, giving both smooth white and 
wrinkled white. 
The remaining individual is ab in appearance and 
abab in constitution, and breeds true to the wrinkled 
white type. 
The expected behaviour of all these different types 
can be followed out by the aid of suitable breeding 
experiments, and not only has this been done in the 
case of the cross which we have been considering, but 
precisely similar phenomena have been shown to be 
taking place for a large number of other characters in 
many different species of plants and in a good many 
animals as well. 
We are now in a position to state the important 
proposition known as Mendel’s law, which is to the 
following effect : 
The gametes of a heterozygote bear the pure parental 
allelomorphs completely separated from one another, 
and the numerical distribution of the separate allelo- 
morphs in the gametes is such that all possible com- 
binations of them are present in approximately equal 
numbers. (Note that it is impossible for more than 
one member of the same pair of allelomorphs to occur 
together in the same gamete.) 
This is the essence of the great discovery made by 
Gregor Mendel, Abbot of Brunn, and published by 
