20 THE FAMILY AND THE NATION 



plants when self- fertilized do not breed true. Out of 

 about a thousand plants of the third generation, almost 

 exactly three-quarters were tall, but one quarter were 

 short. Next year the seeds of this generation were 

 sown, and it was found that the dwarf type bred true 

 and remained fixed in future generations. But the 

 plants of the tall kind differed among themselves. One- 

 third yielded only tall offspring, while two-thirds gave 

 plants divided into "tails" and dwarfs in the old ratio 

 of three to one. The one-third are pure dominants, 

 breeding true to any number of generations, while the 

 two- thirds are impure, giving seeds divided in the 

 same ratio as those of the first generation of hybrids. 



Representing the germ cells in each case as of two 

 varieties, we may write as DD the pure-bred original 

 tall parent, in which both varieties of cell carried the 

 dominant character. Similarly, RR will suggest the 

 germinal nature of the original pure-bred dwarf parent 

 in which both factors are recessive. The fertilization of 

 one of these parents by the other can be represented as 

 DD = RR. Since each hybrid so formed must derive 

 one factor from each parent, each hybrid must have the 

 composition DR, and contain both kinds of germ cells. 

 But, since one of them carries the dominant factor, it 

 alone will control the outer appearance of the plant, 

 which is externally indistinguishable from the original 

 pure -bred dominant DD. The hybrids, when self- 

 fertilized, can give DD, DR, RD, and RR, and each 

 of these arrangements is equally probable. DR and 

 RD are indistinguishable, and together make up half 

 the number of the offspring. These results may be 

 tabulated in a pedigree. 



