June, 1922.} QN THE CROSS-0 VER AND INTERFERENCE. 57 , 



55.29. (i, e., 13.51 + 41.69) or 28.18 (i.e., 41.69-14.51), respectively. 

 As the distance between h and d obtained by my experiments is 

 43.48, the order of these factors should be d, v, h. 



Double Crossing-over and Interference. 



As stated above the loci of these factors are arranged on the 

 chromosome in the order h, v, d. The calculated distance between 

 h and d is' 55.20 while the experimental distance is only 42.19. 

 The difference is 13.01. What is the cause of such difference ? By a 

 three point experiment, Morgan 1) and others recognized that the 

 calculated distance is longer than the experimental distance, especially 

 in the case of distance being longer and that such difference is due 

 to the double crossing-over. 



In the germ cell of the Fi plants produced by the conjugation of 

 two different germ cells which have the factors D, V, H and d, v, h 

 respectively, two kinds of chromosomes should be found as illustrated 

 in Fig. I a, b. In the germ cell of Fi plants, the cross-over take 

 place, between the homologous chromosomes. 



Considering the arrangement of these three factors v, d, and h, 

 all kinds of cross-overs are illustrated in Figures II, IV and VI. Fig. 

 II is the cross-over between V and D, and Fig. IV between V and 

 H. These- cross-overs are called the single cross-overs. From the 

 former results two chromosomes as illustrated in Fig. Ill, the one 

 having the three factors, not rolled, variegated and heart-shaped and 

 the other one rolled, not variegated and not heart-shaped. From the 

 latter results two chromosomes as illustrated in Fig. V, the one 

 having respectively three factors not rolled, not variegated and heart- 

 shaped and the other one rolled, not variegated and heart-shaped 

 on the one hand, and rolled, variegated and not heart-shaped on 

 the other. As illustrated in Fig. VI, there is still a case in which 

 the cross-over occurs at two points, i. e., between D and V, and V 

 and H respectively. This cross-over is called the double crossing 

 over and there is formed two chromosomes one of which has three 

 factors, not rolled, variegated and not heart-shaped, and the other 

 not variegated, rolled and heart-shaped as illustrated in Fig. VII. 

 There is, of course, another case in which no cross-over occurs as 

 illustrated in Fig. I a, b. In short, Fi plants must give eight kinds 

 of individuals having the three characters different from each other. 



1) The Mechanism of Mendelian Heredity. 1915. 



