GENETICS — MORGAN 



347 



clarification of this idea it became possible, as more evidence accumu- 

 lated, to demonstrate that the genes lie in a single line in each 

 chromosome. 



Two years previously (1909) a Belgian investigator, Janssens, had 

 described a phenomenon in the conjugating chromosomes of a sala- 

 mander, Batracoseps^ which he interpreted to mean that interchanges 

 take place between homologous chromosomes. This he called 

 " chiasmatypie " — a phenomenon that has occupied the attention of 

 cytologists down to the present day. Janssens' observations were 

 destined shortly to supply an objective support to the demonstra- 

 tion of genetic inter- 

 change between linked 

 genes carried in the 

 sex chromosomes of 

 the female Drosophila. 



Today we arrange 

 the genes in a chart or 

 map, figure 1. The 

 numbers attached ex- 

 press the distance of 

 each gene from some 

 arbitrary point taken 

 as zero. These num- 

 bers make it possible 

 to foretell how any 

 new character that may 

 appear will be in- 

 herited with respect to 

 all other characters as 

 soon as its crossing-over value with respect to any other two charac- 

 ters is determined. This ability to predict would in itself justify the 

 construction of such maps, even if there were no other facts concern- 

 ing the location of the genes; but there is today direct evidence in sup- 

 port of the view that genes lie in a serial order in the chromosomes. 



heterozygous franslocalfon 



FiGDRB 2. — Diagram to illustrato the case when a piece 

 of one chromosome (black) has been translocated to 

 another chromosome (white). In the lower part of 

 the figure the method of conjugation of these chromo- 

 somes is shown. 



WHAT ARE THE GENES? 



What is the nature of the elements of heredity that Mendel postu- 

 lated as purely theoretical units? What are genes? Now that we 

 locate them in the chromosomes are we justified in regarding them 

 as material units; as chemical bodies of a higher order than mole- 

 cules? Frankly, these are questions with which the working genet- 

 icist has not much concern himself, except now and then to specu- 

 late as to the nature of the postulated elements. There is no con- 

 sensus of opinion amongst geneticists as to what the genes are — 

 whether they are real or purely fictitious — because at the level at 



