530 



THE AMERICAN NATURALIST [Vol. XLVIII 



Sterile Anthers and Cretin Flower 

 Wild Type Sterile Cretin SteriU Cretin 



Eepulsion 764 355 345 25 .683 2.6:1 28. 



The linkage value for sterile and light, namely, 4.4 units, 

 is the most accurately determined of those in the second 

 chromosome. The value for light and cretin is about 28 

 units. Using the distance 4.4 between sterile and light as 

 our base line, then, we should find that cretin lies at 4 -f 28 

 or 32 from sterile if the order of genes is sterile, light, 

 cretin ; but if the order is cretin, sterile, light, then cretin 

 should lie at 28 — 4 or 24 from sterile. The value for 

 sterile cretin should approximate either 24 or 32. There 

 is no very small value here as there was in the first chro- 

 mosome, and not such great accuracy is required of the 

 remaining value, since it should be easy to distinguish be- 

 tween 24 and 32. The coupling data for this value gives 

 33 units, which enables us to fix the order of genes as 

 sterile, light, cretin. The following diagram of chromo- 

 some II expresses these relations more clearly. 



S L C 



When crossing over is as free as in the case of sterile 

 and cretin and of light and cretin there should be some 

 double crossing over. That is, crossing over might occur 

 in the section of the chromosome near sterile and light 

 and at the same time another crossover could occur in the 

 section between light and cretin. This occurrence would 

 be readily seen if normal plants heterozygous in any com- 

 bination of these three genes were back-crossed to plants 

 purely recessive in all three. A relatively few plants from 

 such a test would give very valuable information on sev- 

 eral points, while an experiment of a few thousand indi- 

 viduals from such back-cross tests would enable one to 

 discover, through the phenomenon of interference, much 



