Jnno, 1922.1 QN THE CROSS-OVER AND INTERFERENCE. 59 



The total amount of the cross-over between V and D is the sum 

 of the single (4.8) and of the double (8.9) cross-over, of which the 

 value is 13.7. Likewise the cross-over value for V, H, is 35.5 + 8.9 = 

 44.4. 



If the cross-overs occurred independently of each other, the double 

 crossing-over involving both regions D-V, V-H, should take place in 

 6.08 percent of the cases. In other words, the expected double 

 crossing-over without the occurring of interference should be 13.7 X 

 44.4/100 = 6.08^. Yet the experimental cross-over is 8.9 o/ . There- 

 fore, the difference between the expected double crossing-over and the 

 experimental must be due to the interference of the cross-overs. 



The phenomenon of interference was originally deduced by Stur- 

 tevant 1 ) from his experiment with Drosophila melano g aster . It 

 consists of the fact that the occurrence of a cross-over in one region 

 of a chromosome reduces the chance for the occurrence of another 

 cross-over in a different region of the same chromosome. Studying 

 mathematically this phenomenon MuivLER 2) said that the amount of 

 interference is expressed by an index called a " coincidence," which is 

 the ratio of the double crossing-overs actually observed in the experi- 

 menf as compared with that expected. 



It has been recognized that increasing the distance between the 

 two regions in whch the double crossing-over occurred, the coincidence 

 rises gradually to the value of 1, and that when the distance becomes 

 so great that the coincidence reach the value of 1 the interference 

 has entirely disappeared, or the expected double crossing-over have 

 become equal to the experimental. According to the further studies 

 of the phenomenon by Muller 3) , Bridges 4) , and Weinstein 5) , when 

 the two regions are widely separated the intereference reappears. 

 In this case there is actually found a greater number of double 

 crossing- overs than the expected. 



In his experiment concerning three factors, black, purple and 

 curved, locating on the second chromosome of Drosophila melano- 

 gaster, Bridges reported that the experimental double crossing-over 

 is larger than the expected. Bridges said that in his case although 

 the difference between the experimental and the expected is small, it 



1) Journal of Experimental Zoology, Vol. XIV, 1913. 



2) American Naturalist, Vol. L, No. 593, 1916. 



3) American Naturalist, Vol. L, No. 593, 1916. 



4) Journal of Experimental Zoology, Vol. XIX, 1915. 



5) Genetics, Vol. Ill, No. 2, 1918. 



