IX REPULSION AND COUPLING 89 



supposing that the heterozygous plant produces a series of 

 gametes in which the factors are coupled in this way. The 

 intensity of the coupling, however, varies in different cases. 

 Where we are dealing with another, viz. fertility (J^) and 

 the dark axil (Z>) the experimental numbers accord with 

 the view that the gametic series is here 1 5 JrD : i Jui : 

 I /D : 1 5 fd. The coupling is in this instance more 

 intense. In the case of the erect standard {£) and blue- 

 ness (B) the coupling is even more intense, and the 

 experimental evidence available at present points to the 

 gametic series here being 127 ££ wEbweBwiieb. 

 There is evidence also for supposing that the intensity of the 

 coupling may vary in different families for the same pair of 

 factors. The coupling between blue and long pollen is 

 generally on the 7:1:1:7 basis, but in some cases it may 

 be on the 15:1:1:15 basis. But though the intensity 

 of the coupling may vary it varies in an orderly way. If 

 A and B are the two factors concerned, the results obtained 

 in F2 are explicable on the assumption that the ratio of the 

 four sorts of gametes produced is a term of the series — 



■iAB-^Ab-\-aB+ 2, ab 

 7 AB + Ab + aB+ Tab 

 IS AB + Ab + aB+ic, ab, etc., etc. 



In such a series the number of gametes containing A is 

 equal to the number lacking A, and the same is true for 

 B. Consequently the number of zygotes formed contain- 

 ing A is three times as great as the number of zygotes which 

 do not contain A ; and similarly for B. The proportion 

 of dominants to recessives in each case is 3 : i. It is 

 only in the distribution of the characters with relation to 

 one another that these cases differ from a simple Mendelian 

 case. 



In the account given in the foregoing chapter we treated 

 of repulsion between two factors as being complete, and 

 for practical purposes this is generally true. Nevertheless 

 the most recent work indicates that repulsion like coupling 

 is partial, and the reasons for taking this view may best be 



