ARCHITECTURE OF THE GERMPLASM 245 



7. THE ARRANGEMENT OF THE GENES 



Morgan assumes that if one per cent of cross-overs 

 occurs this may be made to represent one arbitrary 

 unit of distance between the two genes in question. 

 Haldane proposes to call this unit of cross-over a 

 morgan. In the illustration of black-body and vestigial- 

 wing where there was 17% of cross-over it is assumed 

 that the genes for these two characters are 17 units, or 

 morgans, apart in the chromosome. 



Following up this fertile idea it becomes possible 

 even to map the location of the genes in the chro- 

 mosomes. Sturtevant was the first to make such a 

 map for the genes in the "sex chromosome" of Droso- 

 phila. 



This has been followed by maps of the other chro- 

 mosomes, after breeding a total of several million flies 

 and analyzing the data which include . altogether the 

 behavior of over a hundred different genes. 



The relative location of the genes has been determined 

 by the following method. If for example two genes, 

 A and B,, upon breeding back to the recessive show 5% 

 of cross-overs with a and b, while B and C show 20% 

 with their allelomorphs, b and c, then when A and C 

 are bred together with a and c, they should give 

 either the sum (5 + 20 = 25%) or the difference 

 (20 5 = 15%) of cross-overs. 



For example, in an actual experiment, yellow-body 

 and white-eye gave 1.2% cross-overs while white-eye 

 and bifid-wing gave 3.5% cross-overs. When yellow- 

 body and bifid-wing were tested they met the expecta- 



