CHAPTER XV 

 PARTHENOGENESIS AND PURE LINES 



In so far as parthenogenetic reproduction takes place 

 without reduction in number of the chromosomes, the 

 expectation for any character is that it will have the same 

 frequency distribution in successive generations, because 

 the chromosome group is identical in each generation. 

 There are a few cases where parthenogenetic inheritance 

 has been studied. The results conform to expectation. 



The only difference between a species reproducing by 

 diploid parthenogenesis and one propagating vegetatively 

 is that in the latter a group of cells starts the new genera- 

 tion and in the former only one cell, vis:., an egg, that no 

 longer undergoes reduction, or needs to be fertilized. In 

 both, the chromosome complex remains the same as in the 

 parent. Strictly analogous to the tw^o foregoing methods 

 of propagation are the cases of sexual reproduction in a 

 homozygous group of individuals, composed of males and 

 females or in a group of hermaphroditic forms that are 

 homozygous. Successive generations are here also 

 expected to have the same frequency distribution, whether 

 selected or not, because they have the same germ-plasm. 

 Johannsen's pure lines furnish an example of the last 

 case, for, in principle, pure lines, parthenogenetic repro- 

 duction, and vegetative propagation, are concerned with 

 nearly the same situation. 



Johannsen worked with one of the garden beans 

 {Phaseolus vulgaris) taking the w^eight of the seeds, in 

 some cases, and measuring their sizes in other cases. It 

 is known that this bean regularly fertilizes itself. As a 

 consequence of self-fertilization there is a tendency for 

 the descendants of any form to become in time homozy- 

 gous, even when heterozygous forms were present at first. 



204 



