164 ANIMAL LIFE AND HUMAN PKOGKESS 



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

 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 linkage of characters. The cases are 

 peculiar 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 | r Red 

 Long J iRour 



PUT pie Long 



Purple Purple Red 



Loug Tloui[id Long "Routtd 



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



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 Mendelian nature. The line 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 peculiar feature of the 

 case lies 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 



