ix REPULSION AND COUPLING 85 



latter. 1 When a cross is made between a purple 

 with round pollen and a red with long pollen the 

 F x plant is a long pollened purple. But the F 2 

 generation consists of purples with round pollen, 

 purples with long pollen, and reds with long pollen 

 in the ratio I : 2 : i. No red with round pollen 

 appears in F 2 owing to repulsion between the factors 

 for purple (B) and for long pollen (Z,). Similarly 

 plants produced by crossing a red hooded long with 

 a red round having an erect standard give in 

 F x long pollened reds with an erect standard, and 

 these in F 9 produce the three types round pollened 

 erect, long pollened erect, and long pollened hooded 

 in the ratio I : 2 : I. The repulsion here is between 

 the long pollen factor () and the factor for the 

 erect standard (). 



Yet another similar case is known in which we 

 are concerned 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 

 Fj plants are all fertile with dark axils. But such 

 plants in F 2 give fertiles with light axils, fertiles 

 with dark axils, and steriles with dark axils in the 

 ratio i : 2 : i. No light axilled steriles appear from 



1 It should be mentioned that as the shape of the pollen coat, 

 like that of the seed coat, is a maternal character, all the grains of any 

 given plant are either long or 'else round. The two kinds do not 

 occur together on the same plant^. 



