446 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



In the plant, Primula sinensis, Gregory observed the occurrence 

 of linkage in a group of five characters, viz., 



Dominant Recessive 



1. Short style vs. long style (1) 



2. Magenta corolla vs. red corolla (r) 



3. Tinged corolla vs. full-colored corolla 



4. Green stigma vs. red stigma (s) 



5. Pale stem vs. full red stem 



Altenburg later determined the strength of the linkage existing 

 between three of these five pairs of characters, viz., i, 2, and 4 of the 

 above Ust. His results may be expressed in a linkage map as follows : 



1 r s 



o 34.0 45.6 



The cross-over percentage between 1 and r was found to be 34.02, 

 between r and s, 11.62. The sum of these two, 45.64, is the total 

 (uncorrected) mao distance. The observed cross-over -percentage 

 between 1 and s was 40.6, which falls short of the map distance by 

 almost exactly the amount indicated by Haldane's table. 



In the sweet pea the earliest discovered examples of linkage are 

 found. Here are known two linkage groups containmg each three 

 pairs of characters as follows : 



Dominant Recessive Dominant Recessive 



" I. Blue vs. red flower color f i. Dark vs. light leaf-axil 



Group I ■ 2. Long D5. round pollen Group 2 < 2. Fertile j)5. sterile anthers 



3. Erect vs. hooded standard [ 3. Normal vs. cretin flowers 



Results described by Bateson and by Punnett indicate that in Group i 

 the map relations of the three genes are: 



E— B L 



0.78 12.S 



The group is a compact one, with E and B very closely linked, cross- 

 over percentage less than one, with B and L showing between 1 1 and 

 12 per cent crossovers, and with E and L showing about 12.5 per cent 

 of crossovers. 



In Group 2, the cross-over percentage between D and F is about 

 6.2, between F and N about 25.0. Until the cross-over percentage 

 between D and N has been experimentally determined, it cannot be 

 stated whether the "map" order is FDN or FND. In the former 

 case, the total map distance will be 25, or about double the length of 

 Group i; in the latter case, it will be still longer, or about 31.2. 



I 



