164 ANIMAL LIFE AND HUMAN PEOGRESS 



questions of the present, for out of them some day must 

 come the progress of the future. 



Linkage. — One of the earliest departures from the then 

 recognised scheme of Mendelian inheritance was found in 

 what is often termed Unkage of characters. The cases are 

 pecuUar in that the orderly distribution of the factors among 

 the gametes is disturbed in an orderly manner. Purple flower 

 colour, for example, is dominant to red in the sweet pea. The 

 sweet pea is also noteworthy in that some plants produce three- 

 pored oval or long pollen while others have round pollen with 



Purple 1 y J Ked 

 Long j ? 1 "Round 



PuTrpIt Long 



Purple Purple Red. Ked 

 Long T^ouTid Long Hound 



(9) (3) (3) (/) 

 IZ I I 3-3 



Fig. 4. — For explanation see text. The numbers in parentheses indicate 

 the proportion of the four forms which would be expected if the case were 

 one of simple MendeUan nature. The hne of figures below gives the actual 

 proportions of the four forms as found by experiment. 



two pores. Of these, long is dominant to round. When a 

 purple long is crossed with a red round (Fig. 4), the offspring 

 are all purple longs. Were the case one of simple Mendelian 

 heredity the offspring of such plants should be of the four 

 types — purple longs, purple rounds, red longs and red rounds, 

 in the ratio 9:3:3:1. As a matter of fact the four expected 

 classes appear. Further, the ratio of purples to reds is 3 : 1, 

 as also that of the longs to rounds. The pecuhar feature of the 

 case hes in the distribution of the four classes taken together. 

 The purple longs are about twelve times as numerous as the 

 purple rounds, while on the other hand the red longs are barely 



