22 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



The one combination which was sought in this cross is type No. 1 

 of Table 11. This type constituted only 1 sixty-fourth of the second 

 generation of this hybrid. It is the combination WWNNCC, which 

 is pure winter wheat, nonlodging, with tightly closed chaff. The most 

 undesirable type, wwnncc, also occurred once in sixty-four times — that 

 is, it was a spring wheat which lodged and had weak chaff. Further 

 mention of this new type of wheat will be made later in discussing 

 the application of the principles to plant breeding. 



We may now state the law of recombination as follows : In the sec- 

 ond generation of a hybrid there tends to occur every possible com- 

 bination of the original parent characters. 



We may further add to this law that every one of these combina- 

 tions will, if the second generation is numerous enough, occur in some 

 individuals in homozygote form, and will thus be firmly fixed and 

 reproduce true to seed. 



Although all the possible combinations will occur in the second 

 generation of a hybrid (that is, provided the second generation is 

 numerous enough to permit them to occur) unfortunately most of 

 them will be mixed with other combinations that have the same 

 external appearance but very different hereditary tendencies. This 

 is due to the fact of dominance. For instance, the homozygote form 

 WWNNCC of Table II can not be told by inspection from the form 

 just following it (WWNNCc) or from several other of the twenty- 

 seven combinations. One way to overcome this difficulty in a self- 

 pollinated species is to save the seed of each 'second-generation plant 

 separately. If the species is one that is not self-fertilized, but one 

 which can be artificially self-fertilized, we can accomplish the segre- 

 gation of the desired type by artificial self-fertilization of all the sec- 

 ond-generation individuals that appear to be of the type desired and 

 by planting their seed separately. When the next generation matures 

 it will be seen which of them have reproduced true to type. The 

 seed of these may be saved, and thus form the basis of a new and 

 fixed variety in self -fertilized species and in such open-fertilized 

 species as will endure such self-fertilization. 



A very beautiful illustration of the law of recombination is seen in 

 the work of Professors Price and Drinkard, of the Virginia Agricul- 

 tural Experiment Station, in their experiments with hybrid tomatoes. 

 Two varieties were crossed which differed in three respects, namely, 

 one variety had green leaves, the other yellowish green ; one had red 

 fruit, the other yellow; one had pear-shaped fruit, the other round. 



"The writer discovered this law independently in 1901. See Bulletin No. 115, 

 Office of Experiment Stations, U. S. Dept. of Agriculture. 

 165 



