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, 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 

 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 23, 

 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 F 2 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 



c r p 



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. 1 ] 



1 See Genetics and Eugenics. 



