MENDEL'S FIRST LAW 21 



others produced short plants; in the ratio of 3 tall to 1 

 short. In other words, the contrasted characters of the 

 grandparents reappeared in the grandchildren in the ratio 

 of 3 to 1. The experiment was carried through one more 

 generation, which was necessary in order to get data 

 for finding out what had been taking place. The short 

 peas were allowed to fertilize themselves. They pro- 

 duced only short peas. The tall peas were also allowed to 

 fertilize themselves. One-third of the tall peas produced 

 only tall offspring; two-thirds produced both tall and 

 short offspring in the ratio of 3 : 1, as had the first genera- 

 tion hybrids. Evidently then the grandchildren had been 

 of three kinds, one kind was pure for shortness, others 

 were hybrids, and the remaining kind was pure for tall- 

 ness. These kinds appeared in the proportion of 1 : 2 : 1. 

 Some factor or factors in the original tall peas must 

 cause the peas of that race to be always tall, and some 

 factor in the original short peas must cause them to be 

 short. The short factor may be represented by s, and 

 the long factor by S. When crossed, the fertilized egg 

 should contain both factors (sS), and since the hybrids 

 coming from this egg were tall, it is evident that tall must 

 dominate over short. Now if the two factors (sS) present 

 in the hybrid should separate {i.e., "segregate'') when its 

 ovules and its pollen-grains are formed, half of the eggs 

 would contain the factor that represents the short peas 

 (5), and half of the eggs the factor that represents tall 

 peas (S) ; also half of the pollen grains would contain the 

 factor that represents the short peas (5), and half of them 

 would contain the factor that represents the tall peas (S). 

 Chance meeting between egg-cells and pollen-cells (one 

 ovule being always fertilized by one pollen grain), would, 

 on the average, give one fertilized egg containing two 

 factors for short (55) ; to two fertilized eggs that contain 

 one of each kind of factor (sS) ; to one that contains two 



