INTERFERENCE 129 



each other, take place in 0.6 per cent, of the cases. But 

 if the regions in question are close together, that is, if the 

 intervening block (i.e., G F E J K L) oi genes is short, it 

 is found that there are fewer double crossovers than the 

 0.6 per cent, expected on a purely random basis. This 

 was shown by Sturtevant in his paper on chromosome 

 maps. It means that a break in one region interferes 

 with a break in the other region when the intervening 

 block is short. 



The ratio of the number* of actual double breaks 

 obtained to the number of double breaks that would occur 

 if one of them did not interfere with the other is termed 

 coincidence. If in the above example only 0.3 per cent, 

 of the cases were double crossovers involving the regions 

 ABCDEF SiTid MN0PQR8T the coincidence would 

 be 0.3 per cent, divided by 0.6 per cent., or 0.5. 



It has been found that as the distance between two 

 regions increases, crossing over in one of them interferes 

 less and less with crossing over in the other; that is, the 

 number of double crossovers obtained approaches the 

 number expected on a random basis, and coincidence rises 

 gradually to the value of 1. This phenomenon is shown 

 in all the cases where more than one block of genes has 

 been followed. It is especially clear in the work of Muller, 

 who studied a large number of factors in the sex-chromo- 

 some of Drosophila simultaneously. 



When the intervening block becomes sufficiently long 

 so that the coincidence attains the value of 1, interference 

 has entirely disappeared. When, however, the distance is 

 increased still further interference reappears, i.e., coin- 

 cidence decreases again. There was a suggestion of this 

 in Muller 's work; and the work of Weinstein undertaken 

 to get critical evidence on this point indicates clearly that 

 such a decrease exists. For the second chromosome a 

 similar rise and fall with increase of distance is indicated 

 by Bridges ' data. 



9 



