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



througli a rearraiigenieiit of substances iu the region of tiw 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,c chromo- 

 somes. The one important difference between the two methods of 

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

 determiners, for by the method illustrated in figure 6, in which a whole 

 chromosome is displaced, a mechanism is provided which would preserve 

 any coupling which had existed through the association of several genes 

 in the same chromosome. The method illustrated in figure 7 would 

 favor the breaking of such , ^ < . 



a coupled system. Some r 

 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 detei-- \_^ 

 miners from single ones seem > 2 j ., 



Verv plausible If the Chro- ^'^' "• Hypothetical duplication of a determiner 



mosomes are the bearers of Y ' ^'^Sitn^^'^^^ transfer of a gene from one 



,. ,. , cliromosome to an adjacent one. Significance of 



the Mendehan genes, the ,y„,,„l, t,,^ ,^^^ ^^ i^ g^ ,, 



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 di\äsion 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 wliich 

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

 result not only iu the duplication of tliis 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 ])roduction this gene is an essential 



10* 



