PARTHENOGENESIS AND PURE LINES 205 



In fact, in a few generations perpetuated by self-fertiliza- 

 tion with chance elimination of individuals, a homozygous 

 race will result. This comes about as follows : Starting 

 with a heterozygous hermaphroditic individual, some of 

 its offspring will, through recombination of factors, 

 become homozygous, and if self-fertilization prevails they 

 will continue homozygous ; other offspring will be hetero- 

 zygous. From the latter both homo- and hetero-zygous 

 oifspring will again be produced, the former remaining 

 such in later generations, the latter continuing the process 

 of splitting. Since only a part of each generation sur- 

 vives, there is in the long run a better chance that the 

 homozygous individuals will be the survivors, because 

 those that have become such in each generation are fixed, 

 and those that are not will continue to produce some 

 homozygotes. There will be in consequence a steady proc- 

 ess of recurrence of homozygotes which, on chance alone, 

 will sooner or later win out. 



The beans that Johannsen worked with had apparently 

 reached a homozygous condition, and at the start there 

 must have been several such lines. He studied nineteen 

 of them. The oifspring of any one plant produced beans 

 that gave the same frequency distribution as the beans 

 of the last generation. This condition continued through 

 all successive generations. It is to be noted that the beans 

 on any one plant differ in size, but any one will give the 

 same frequency distribution as the beans of the preceding 

 generation. It made no difference whether the larger 

 or the smaller beans were chosen for planting — they gave 

 the same group in the next generation. 



It is interesting to compare this result with what would 

 have happened had the beans been propagating by cross- 

 fertilization at the time when Johannsen began his work 

 with them. If this had been their normal method of 

 reproduction they would probably have been heterozygous 

 at the start, and would have given different genetic types 

 for several generations, even if self -fertilized. Pure lines 



