EXTRA CHROMOSOMES — BI^VKESLEE 445 



By the proper breeding; priK-cilinv tlii'se types, which are hybrid for 

 the modified chromosoiues and therefore show circles of four, may 

 be i)iirilied so that the new tertiary chn)niosonies are in pairs. Such 

 })urilied races witli modified chromosomes are called ptivie types. 

 By crossing such prime types onto the })roper (2n + l) forms, it is 

 possible to get the modified chromosome as an extra and thus to form 

 (2n+l) tertiary forms. At the present time we have 40 different 

 j)rime tyi)es and about 90 types with circles or chains which we are 

 attempting to j)urify into prime types. 



Translocations have been also induced by radiation. In a speiific 

 case the 1'2 chromosome was broken in two and the 12 half was 

 translocated and permanently attached to the 11 end of the 11 • I'i 

 chromosome making a large compound chromosome 2 "11 'IS. The 

 1 portion was left free as a fragment. AVe have gotten rid of the 

 1 fragment and obtained plants with one and others with two of 

 the 2 • 11 • 12 chromosome. The latter breed true and have given rise 

 to a distinct race very ililferent in a[)pearance from normal 2n plants 

 from which they arose. 



Compensating types nui^' be obtained in various ways such as by 



crossing a i)rime type onto the appropriate secondary (2n+l) form 



and obtaining the comi)ensating type in the next generation. In the 



otfspi-ing of a plant with a circle of six attached chromosomes, one 



shouhl expect from one to three compensating types according to the 



manner in which the abnormal chromosomes have been modified. 



Thus radiation treatment induced a circle of six witli tlie followiuir 



composition : 



11-12— 12 17— 17 18 



I I 



1113— 1;M4— 14 18 



In the next generation a compensating type was obtained in which 

 the 11 • 13 and the 12 • 17 tertiaries compensated for the 11 • 12 normal 

 chromosome. 



Circles of six and more chromosomes may be obtained as a source 

 of compensating types by crossing two appropriate prime types 

 together. Thus we have two prime types (a) and {h). The for- 

 mula for {a) is: 1*14, 2-13, 23-24. The formula for (5) is 1-2, 

 13-23, 14-24. "When either is crossed with our standard line 1, 

 a circle of four results as shown in a preceding i)aragrai)]i. When 

 (a) is crossed with {h) the hybrid which results has a circle of six. 

 The formula of this circle would be as follows: 



12 — 2 13— 13 23 



I I 



114— 14-24— 24-23 



By crossing prime types together we have obtained circles composed 

 of as many as 10 chromosomes. The larger the number of chronio- 



