GENETIC CHANGES 215 



showing the possibility of the occurrence of genetic change 

 outside of sexual reproduction.^ 



Self-fertilization among plants is almost as favorable as 

 parthenogenesis or as vegetative reproduction for showing 

 genetic changes, if they occur. For in self-fertilization both 

 egg and pollen gametes are furnished by the same parent 

 individual. Johannsen first advanced the view that when 

 such a parent individual is homozygous for all genetic 

 factors, no genetic changes will be observed among the de- 

 scendants, which will continue generation after generation 

 to constitute a '*pure line." He substantiated this view by 

 studies of size variation in successive generations of self- 

 fertilized beans. He found in a number of cases that no 

 change in size resulted from selecting in successive genera- 

 tions either the largest or the smallest beans borne on the 

 same mother plant and concluded that such plants were 

 homozygous for all genetic factors affecting size of seed, and 

 that the observed variations in size upon which his selec- 

 tions had been based were due to environmental agencies 

 such as the position of the bean in the pod and the consequent 

 amount of material available for storage in the seed, which 

 conditions were not subject to inheritance. 



The case is very different if one selects by size beans borne 

 on a plant heterozygous for genetic size factors (as for ex- 

 ample an Fi plant from a cross between a large-seeded and 

 a small-seeded race of beans). Under those conditions races 

 differing in average seed-size are quickly segregated (Emer- 

 son) . Johannsen's observations show that genetic variations 

 affecting the seed-size of beans are not of frequent occur- 

 rence, yet he has himself recorded the occasional occurrence 



^ A very puzzling case of genetic change in parthenogenesis is recorded by Na- 

 bours. He observed in grouse locusts (Apotettix) the development of offspring from 

 unfertilized eggs which showed unmistakable segregation of characters and even 

 crossing-over among linked characters for which the mother was heterozygous. All 

 the offspring, however, were of the female sex, indicating that the eggs from which 

 they developed had not undergone reduction as regards the sex determinant, thougii 

 it would appear that they must have undergone reduction as regards other char- 

 acters. Cytological study of such material should prove interesting. 



