306 LOCK : STUDIES IN PLANT BREEDING 
separate allelomorphs of the parents are represented in the 
gametes in all possible combinations in eqnal numbers, with 
the exception that the two allelomorphs of the same pair 
never occur together in the same gamete. To this principle 
Correns (11) has applied the term “ Mendel’s Law.” 
Thus if the parental allelomorphs be represented by A and 
A' respectively, they will give rise to a heterozygote contain¬ 
ing both allelomorphs—AA'; but the gametes of this will 
contain the pure parental allelomorphs A and A', moreover 50 
per cent.of the gametes will contain A but not A', and 50 per 
cent, will contain A' but not A. If we pay attention to two 
pairs of characters—A & A', B & B' (AB in one parent, A'B' in 
the other), the heterozygote will contain A ABB'. This will 
produce equal numbers of gametes : 25 per cent. AB, 25 per 
cent. AB', 25 per cent. A'B and 25 per cent. A'B'; but there 
will be no gametes of the form A A' or BB'. 
It is found, therefore, on self-fertilizing a heterozygote 
formed by the union of two gametes which contain the 
allelomorphs A and A' respectively (and whose own gametes 
are therefore 50 per cent. A, 50 per cent A' in each sex), that 
offspring of the forms AA, AA', A'A and A'A' appear in equal 
numbers. If, on the other hand, the gametes of the heterozy- 
gote AA' are crossed by the gametes of the pure parental 
form A' (all gametes A'), the offspring will show the forms 
AA' and A'A' in equal numbers. In the particular cases 
studied by Mendel AA, AA' and A'A were similar in 
appearance, so that in the first of the above cases the propor¬ 
tion in which the offspring appeared was apparently 
3 A: 1A' or 75: 25; and in the second case 1A : 1A' or 50: 
50. These characteristic proportions are sometimes spoken 
of as Mendelian ratios. 
We may now pass on to consider the experimental evidence 
upon which the above conclusions are based. 
(1) Mendelian Monohybrids. 
These are heterozygotes formed by the union of gametes 
m whichtheallelomorphs of a single pair only are considered. 
