174 GENETICS AND EUGENICS 



showing between 11 and 12 per cent crossovers, and with 

 E and L showing about 12.5 per cent of crossovers. 



In Group II, 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 F D N or F N D. 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 (Wliite). In one of 

 the established 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 flow- 

 ering 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 crossovers occurring. 



In maize three linkage groups are known, one of four 

 factors and two of two factors each. Group 1 includes a 

 factor for waxy endosperm and the factor C for aleurone 

 color. These show a cross-over percentage of 26.7. Group 2 

 includes fowr linked factors, aleurone factor R, chlorophyl 

 factor G, chlorophyl factor L, and aleurone spotting factor, 

 S. No crossovers have been observed between R and L 

 which behave as if they were allelomorphs, or ** completely 

 linked." The cross-over percentage between L and G has 

 been determined as 23, that between R and G has been de- 

 termined less accurately as 19, and that between R and S 

 as 12.5. The order of the genes is accordingly 



RL S G. 



Group 3 includes the two characters, starchy endosperm 

 and tunicate ("podded") seeds. The cross-over percentage 

 in this case is 8.3 (Jones and Gallastegui). 



In the cultivated tomato two cases of linkage have been 

 reported. A gene for "standard" vine habit and a gene for 



