ix REPULSION AND COUPLING 93 



Yet another similar case is known in which we are con- 

 cerned with quite different factors. In some sweet peas 

 the axils whence the leaves and flower stalks spring from 

 the main stem are of a deep red colour. In others they 

 are green. The dark pigmented axil is dominant to the 

 light one. Again, in some sweet peas the anthers are 

 sterile, setting no pollen, and this condition is recessive 

 to the ordinary fertile condition. When a sterile plant 

 with a dark axil is crossed by a fertile plant with a light 

 axil, the FI plants are all fertile with dark axils. But 

 such plants in F 2 give fer tiles with light axils, fer tiles with 

 dark axils, and steriles with dark axils in the ratio 1:2:1. 

 No light axilled steriles appear from such a cross owing 

 to the repulsion between the factor for dark axil (D) and 

 that for the fertile anther (F). 



These four cases have already been found in the sweet 

 pea, and similar phenomena have been met with by 

 Gregory in primulas. To certain seemingly analogous 

 cases in animals where sex is concerned we shall refer 

 later. 



Now all of these four cases present a common feature 

 which probably has not escaped the attention of the 

 reader. In all of them the original cross was such as to 

 introduce one of the repelling factors with each of the two 

 parents. If we denote our two factors by A and B, the 

 crosses have always been of the nature AAbbxaaBB. 

 Let us now consider what happens when both of the 



