448 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



In garden peas two independent pairs of linked characters are 

 known and two more are suspected (White) . 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 flowering is linked with red flower color with an 

 estimated cross-over percentage of between 12 and 16. 



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

 found by Baur to be Unked, 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 

 factor R, chlorophyll factor G, chlorophyll 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 27,, 

 that between R and Ghas been determined less accurately as 19, and 

 that between R and S as 12.5. The order of the genes is accordingly 

 R— L 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 "constricted" fruit 

 shape show about 20 per cent of crossing over. In another linkage 

 group, no crossovers have been observed between green foliage color 

 and two-celled fruit, as opposed to yellow foliage color and many- 

 celled fruit, in a total of 24 F2 plants. It seems probable that the 

 linkage in this latter case is close, though the number of observations 

 is too small to do more than establish a probability. 



In rats a group of three linked characters has been found, albinism 

 (c), red-eye (r) and pink-eye (p), which may be mapped, thus 



01 20 



In mice albinism (c) and pink-eye (p) are linked, as they are in rats, 

 but the cross-over percentage is less, viz., 14.3, [ Castle cites further 

 examples.'] 



' See Genetics and Eugenics, 



