66 CELL HEREDITY 



dimensional structure. If linear, we can immediately eliminate the possi- 

 bility of the order 



15 per cent 



arg thi paba 



30 per cent 



because the distance between arg and thi (43 per cent) is greater than 

 that between arg and paba. If the order were 



15 per cent 



paba arg thi 



30 per cent 



then the distance between arg and thi should be 15 per cent. But it was 



not; rather it was 43 per cent. The only spatial arrangement consistent 



with these data is: 



15 per cent 30 per cent 



A. A 



arg paba thi 



43 per cent 



because 43 is not significantly different from 45 per cent, the sum of 15 

 and 30 per cent. Arrangements other than linearity have been ruled out 

 by the mapping of scores of genes in sequence in this way: the distances 

 between them are always approximately additive. Thus genes are ar- 

 ranged in a linear order and the chance of recombination between them 

 is fairly constant and a function of distance. 



The method of mapping provides an image of the material basis of 

 heredity as a linear structure like a rod along which the genes are 

 located in order at specific loci. This image fits very well with the 

 knowledge that the gene is a nucleic acid molecule which is also a linear 

 structure, differentiated chemically along its length. It also fits the 

 knowledge that the vast majority of known genes are located on chromo- 

 somes; these too are linear structures made, in part, of DNA. 



GENES ON CHROMOSOMES 



The chromosomal basis of Mendelian inheritance was established in 

 the decade 1910-1920, primarily by Morgan and his students working on 

 Drosophila, at Columbia University. Later a system for demonstrating 

 more directly the connection between genetic and cytological behavior 

 was found in the red bread mold Neurospora. 



