316 LOCK : STUDIES IN PLANT BREEDING 
seven pairs of characters according to the formula (A+2Aa 
+a) (B + 2 Bb + b) (0 + 2 Cc + c) Ac. 
In the case of the parchment layer of the pod, I found the 
heterozygotes from a cross between Sutton’s French Sugar 
Pea and my native yellow variety to show a slight approach 
to the recessive character, the stiff parchment layer being to 
some extent wanting from certain areas of the pod. The 
appearance, however, was much nearer to that of the 
dominant, and from a little distance such pods appeared 
fully inflated. 
(4) The Theory of Purity of the Gametes. 
Mendel’s explanation of his facts with regard to peas is as 
follows :—He supposes the pollen grains and ovules, or, as we 
should now say, the germ cells of the hybrid plants (heterozy¬ 
gotes), to contain each of them only one member of every pair 
of allelomorphs, and that with this limitation all possible 
combinations of the various allelomorphs occur in the 
gametes in equal numbers. Then, if the male and female 
gametes meet simply according to the laws of chance, we 
shall obtain the observed proportion of offspring showing the 
various characters. This supposition explains the facts, and 
no other hypothesis has so far been advanced which does so. 
This is the essential part of Mendel’s discovery, and it is to 
this principle that the term “ Mendel’s Law” was applied by 
Correns in 1900. 
Thus in the case of a monohybrid, A x a, the heterozygote 
Aa (F,) produces gametes A and a. These combine in all 
possible ways, AA, Aa, aA, aa, with the result AA + 2Aa + 
aa, or A + 2Aa + a. 
In the case of a dihybrid, ABab, the gametes AB, Ab, 
aB, and ab, arise in Fl And these form the combinations 
ABAB + AbAb + aBaB + abab + 2ABAb + 2ABaB + 
2ABab 4- 2AbaB 2Abab + 2aBab, or (omitting dupli" 
cations) AB + Ab + aB + ab -»- 2AaB + 2AbB + 2Aab + 
2aBb + 4AaBb = (A + 2Aa x a) (B + 2Bb + b), and so 
on for more complicated cases. 
