INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1191 



completely his assumptions, and although the foregoing observations on 

 the cytological maps have proved the main corollary of Morgan's theory, 

 segmental interchange as the cytological basis of crossing over had still 

 to be inferred from the genetic data. The experiments of Stern (128) on 

 Drosophila and of Creighton and McClintock (21) on maize have filled 

 this gap. 



Stern (128) used in his study two translocations in Drosophila melano- 

 gaster, one involving the X- and the F-chromosomes, and the other involv- 

 ing the X- and the fourth chromosomes. In the former, a section of the 

 Y is attached to the spindle attachment end of the X, the X-chromosome 

 appearing cytologically as J-shaped instead of rod-shaped as is normally 

 the case. In the latter the X was broken between the genes forked and 

 bar (cf. Fig. 4), the left fragment (y-f) was attached to the fourth chromo- 

 some, and the right one {B-bb) is free. Individuals carrying this trans- 

 location have instead of the normal X-chromosome two rod-shaped 

 fragments of a length equal to about one-half of the normal X, and have 

 only one instead of two free fourth chromosomes (since the other fourth, 

 to which a fragment of the X is now attached, appears rod-shaped). 



Stern marked the chromosomes involved in these translocations by 

 appropriate genes and obtained females which were heterozygous for both 

 translocations simultaneously. Such individuals have instead of the 

 two normal X's one J-shaped chromosome (the X with the fragment of 

 the Y attached to it), two short rod-shaped chromosomes (the broken X 

 of the X-IV translocation), and one free fourth chromosome. These 

 females were crossed to cytologically normal males. In the offspring 

 some individuals were cross-overs between / aijd bb (cf. Fig. 4). It is 

 easy to see that there must be two types of such cross-overs, which are 

 easily distinguishable genetically, by observing the marking genes they 

 carry. In one type the left end of the chromosome of the X-Y trans- 

 location is combined with the right end of the right fragment of the X-IV 

 translocation. The resulting chromosome must appear as a normal 

 rod-shaped X-chromosome, and individuals carrying it must also carry 

 two normal fourth chromosomes. Stern investigated cytologically a 

 large number of individuals of this type and found that they had the 

 expected type of chromosomes. 



The other type of cross-overs, likewise distinguishable genetically, 

 must have the broken X of the X-IV translocation, but the right fragment 

 must acquire the spindle fiber end of the X of the X-Y translocation, to 

 which the fragment of the Y is attached. The resulting individuals 

 must have, cytologically, one rod-shaped chromosome of the length equal 

 to one-half of the normal X (the left fragment of the X-IV translocation), 

 a small V-shaped chromosome present in neither translocation nor in the 

 normal flies (the right fragment of the X-IY to which the section of the 

 F-chromosome is now attached), and one free fourth chromosome. 



