CYTOLOGY AND MENDELIAN HEREDITY 



183 



when present lies very close to the terminal knob. In addition to the 

 above three classes there was a single individual that was partly normal 

 green and partly yellow-green. In the normal-green portion of the plant 

 the ninth chromosome pair was unaltered, as in the first class, whereas 

 in the yellow-green portion the paternal chromosome was shortened and 

 had no knob. Evidently, therefore, the deletion in this case was truly 



Yellow -green 



i t 



Eggs 



Normal green 



Sperms 

 (irradiated! 



, I 



yg. 



Yg, 



y<3t 



Normal green 



Yellow-green 

 I 



Majority of — . n ,^ . . . 



*w \\ J- Deleted 



the plants — 



Yellow -green 

 Deleted 



-K 



Partly normal 

 and partly 

 yel low-greerv 



Terminal deletion 

 in embryo 



Fig. 129. — Diagram illustrating the method of determining gene location by the use of 

 deletions. Explanation in text. {Based on work of H. B. Creighton.) 



terminal, a rare condition; moreover, it must have occurred in the 

 embryonic stage of the plant, since only a portion of the plant was 

 affected. This plant was of further interest from the standpoint of the 

 physiology of genie action, for the particular effect of each of the two 

 genotypes was restricted to the portion of the plant containing it. 

 Relatively few exceptions to this type of genie action are known. 



Gene location by the deletion method is most precisely determinable 

 in organisms having chromosomes that are large and distinguishable on 



