XI COUPLING AND REPULSION 125 



form of flower known as the cretin (Fig. 1 7) which 

 is recessive to the normal flower. The factors for 

 normal (TV) as opposed to cretin («) and fertile 

 pollen (F) as opposed to sterility (/) exhibit the 

 phenomenon of coupling and repulsion. When a 

 normal sterile is crossed with a cretin fertile, the 

 F^ plants are all normal fertiles. 



Normal sterile x Cretin fertile 



I 

 Normal 

 fertile 



I I I i 



Normal Normal Cretin Cretin 



fertile sterile fertile sterile 



336 150 143 II Actual numbers. 



7J0 ijo ijo 10 Expectation on 



1:3:3:1 basis. 



In F^ the four possible combinations appear, 

 but in proportions suggesting that the gametic 

 series formed by the F^ plant is of the nature 

 I NF : 3 N/: 3 nF : i n/. Such a series would 

 give the four classes in the ratio 33 : 15 : IS : i — a 

 ratio closely fulfilled by the experimental results as 

 is shown in the appended scheme. Further, when 

 the normal fertile Fj is produced by the union of a 

 normal fertile gamete with a cretin sterile one 

 (NF X nf) coupling occurs in F^ between Nx F, so 

 that the gametic series must here be supposed to 

 be of the form 3 NF : 1 NF : \ nF : I nf. In the 

 case of these two factors the value of the coupling 

 and repulsion is lower than in the case of the factors 

 for blue and long pollen. It is on a 3 : i instead of 

 on a 7 : I basis. 



In other cases, however, the intensity of the 



