Duplicate genes for capsule-form in Bursa bursa-pastoris. 139 



through a rearrangement of substances in the region of the break 

 between two chromosomes, - - resulting in a sort of "longitudinal cross- 

 over". This would now serve to locate the determiner C in two differ- 

 ent pairs of chromosomes, so that here again as in the preceding case, 

 two chromosomes containing the same gene, C,C, could be included in 

 a single germ-cell, leaving a corresponding germ-cell with only c,e chromo- 

 somes. The one important difference between the two methods of 

 duplication illustrated by figures 6 and 7 relates to the coupling of 

 det^ miners, for by the method illustrated in figure 6, in which a whole 

 chr* ^osome is displaced, a mechanism is provided which would preserve 

 any oupling which had existed through the association of several genes 

 in "i same chromosome. The method illustrated in figure 7 would 

 favor the breaking of such , 2 3 4 



a c< ipled system. Some 

 situation may arise in which 

 this difference may give a 

 clue to the method by which 

 duplication has arisen. 



Both of these methods 

 of origin of duplicate deter- 

 miners from single ones seem 

 very plausible. If the chro- 



Fig. 7. Hypothetical duplication of a determiner 



by a longitudinal transfer of a gene from one 



chromosome to an adjacent one. Significance of 



symbols the same as in fig. 6. 



mosomes are the bearers of 

 the Mendelian genes, the 

 relative rarity of duplication 

 leads necessarily to the inference that, in the first place, homologous 

 chromosomes are very fixed in the relations in which they pair during 

 fertilization, and in the second place, that division of the spireme to 

 form the daughter chromosomes is also very fixed in position. Upon 

 this fixity of relations, both in the formation of the chromosomes and 

 in their subsequent movements must depend, then, the very general 

 occurrence of normal Mendelian inheritance. 



It is of great interest and importance in relation to the duplica- 

 tion of determiners for the triangular capsule in Bursa, to note that 

 either of the two methods just described, or any other process by which 

 a Mendelian gene is carried out of its previous relative position, would 

 result not only in the duplication of this determiner, but, if this gene 

 had not been previously duplicated, the rearrangement would at the 

 same time make possible the production of an individual from which the 

 character is missing, for whose production this gene is an essential 



10* 



