The Bearing of Gytological Research on Heredity. 



347 



KG, in the other WY, usually maintain their original constitution, yet 

 occasionally may undergo an exchange of units so as to produce the new 

 combinations observed. How is this possible? 



E 

 F 

 G 



I-X 



Haploid Group 

 Gamete 



I 



m 



V 



w 

 IF 



Chromosomes 



XX 



Diploid Group 

 Horr.ozygote (9) 



V 

 w 



V 



w 



11 iff m w w 



Chromosome Pairs 



Fig. 4. — Diagram of chromosomes and linkage-groups, based on the relations observed 

 in Drosophila ampelophila. Heavy vertical lines represent chromosomes (or the 

 chromatin-threads from which they arise), letters different factors or gens, assumed 

 to be aligned in linear series in the threads. In the diploid groups corresponding 

 chromosomes are paired side by side in the position assumed by them during 

 conjugation or synapsis. 



Each of the four series A-G, H-N, O-U, V-W, forms a definite linkage-group in 

 which the factors tend to cohere, while independent of all other factors. 



In the male diploid groups one X-chromosome is missing, its place being taken by 

 a Y-chromosome (Lygceus type). The nature of the latter is still unknown. 



In attempting to answer this, it is necessary to bear in mind that the 

 recombinations with which we are dealing affect units that are usually linked 

 together, and hence belong to the same group — in the example just given to 

 the sex-linked or X-group. The recombination or exchange of units must 

 accordingly take place between the corresponding or homologous chromo- 

 somes of a pair (here the X-pair). It therefore becomes probable that the 

 exchange is effected during; the intimate association of these chromosomes in 

 conjugation or synapsis. It was long since suggested by Boveri that an 

 exchange of particular elements might take place between conjugating 

 chromosomes as between conjugating Protozoa, but this suggestion is too 

 vague for our present purpose. The basis for a more specific explanation was 

 offered by Janssens in 1909 in his theory of the " chiasmatype," more recently 



vol. lxxxviii. — ?,. 2 c 



