446 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



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 containing each three 

 pairs of characters as follows : 



Dominant Recessive 



{I. Blue VS. red flower color 

 2. Long vs. round pollen 

 3. Erect vs. hooded standard 



( I. Dark vs. light leaf-axil 

 Group 2 I 2. Fertile vs. sterile anthers 

 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 12.5 



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 END. 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. 



In garden peas two independent pairs of linked characters are 

 known and two more are suspected (White) . In one of the estabhshed 

 cases close linkage is found between round starchy seeds and tendrils 

 on the leaves, with about 1.5 per cent of crossing-over. In the other 

 case a gene for late flowering is linked with red flower color with an 

 estimated cross-over percentage of between 12 and 16. 



In the snapdragon, Antirrhinum, two factors for flower color were 

 found by Baur to be linked, with about 20 per cent of cross-overs occur- 

 ring. 



In maize three linkage groups are known, one of four factors and 

 two of two factors each. Group i includes a factor for waxy endo- 

 sperm and the factor C for aleurone color. These show a cross-over 

 percentage of 26.7. Group 2 includes four linked factors, aleurone 

 fa ctor R, chlorophyll factor G, chlorophyll factor L, and aleurone 



