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THE AMERICAN NATURALIST [Vol. XLVIII 



calculated from the zygotic series corresponding to such 

 gametic ratios as 2.5:1, 3:1, 3.5:1, etc. For the same 

 ratio in the coupling and repulsion series the coefficients 

 are slightly different, so that two tables should be made. 



Upon the chromosome basis the best method of express- 

 ing the amount of linkage is in terms of percentage of 

 crossing over. The gametic ratio u : 1 found through the 

 coefficient of association, when expressed as a percentage 

 100 



becomes ^p-j* 



According to the chromosome hypothesis, all genes 

 which are linked to each other lie in the same chromosome. 

 In sweet peas the first case in which linkage was observed 

 was that of round pollen- and red flower color. Later it 

 was found that hooded standard was linked to round and 

 to red. The genes for these three characters, then, may 

 be treated as though carried by the same chromosome, 

 which we may call chromosome I, of the sweet pea. 



The relative distances of these genes from one another 

 in the chromosome can be determined from the degrees of 

 linkage. The farther apart in the chromosome any two 

 genes lie, the greater will be the amount of crossing over 

 between them. If two genes lie very close together, then, 

 the percentage of crossing-over will be very small (the 

 gametic ratio very large). 



Fortunately Punnett has recently collected the data 

 upon these linkage cases in sweet peas. In the table which 

 follows, I have summarized the data given by the various 

 tables of Punnett. In the first column to the right of the 

 data appear the coefficients of association. In the next 

 column appear the corresponding gametic ratios calcu- 

 lated by interpolation to the nearest tenth. In the last 

 column are the equivalent percentages of crossing over, 



We may use one per cent, of crossing over as our unit 



