MENDEL'S LAW 97 



A comparison of this case with the one just previously 

 described shows what an increasingly difficult thing it is to 

 fix types obtained by crossing, as the number of dominant 

 characters in the selected type increases. On the theory of 

 unit-characters the gametic combinations and segregations 

 in this cross are as shown in Fig. 48. The nature of the 

 gametes formed by the parents crossed is shown in the first 

 row; the composition of the Fi individuals, immediately be- 

 low. In the two lower rows are shown four different sorts of 

 gametic splittings which may occur in Fi individuals, pro- 

 ducing thus eight different kinds of gametes. 



If, as suggested, the Fi individuals produced in this cross 

 form eight different kinds of gametes, each of these kinds 

 should, when united with a gamete having the same consti- 

 tution as itself, produce a homozygous and so true-breeding 

 zygote of a different variety, making in all eight true-breeding 

 varieties. Experiment has shown that in reality eight such 

 varieties are produced in F2. It is therefore evident that the 

 crossing of varieties which differ from each other by unit- 

 characters becomes, under the operation of Mendel's law, a 

 ready means of producing other new varieties different from 

 those crossed, and that the number of such new varieties 

 capable of production in this way increases rapidly with every 

 additional unit-character difference between the parent 

 varieties which are crossed. 



