CHROMOSOME ABERRATIONS IN ANIMALS 677 



X rays, or by exposure of eggs to near-infrared radiation at the time of 

 their fertilization by X-ray-treated spermatozoa; see Kaufmann and 

 Hollaender, 1946, and Kaufmann, 1946b). Potential breaks that fail to 

 establish new contacts during the period of recombination presumably 

 undergo restitution soon thereafter, although some experimental evidence 

 suggests that recombination may be delayed until after the first cleavage 

 mitosis (Heifer, 1940). 



3-lf . Break Recombination. The random distribution of breaks among 

 the chromosomes of Drosophila having been established by analysis of 

 viable rearrangements, the question remains whether breakage ends unite 

 with equal freedom to produce inviable types of exchanges. Informa- 

 tion on this question is available with respect to some of the alternative 

 possibilities of recombination within and among chromosomes. 



Intrachromosomal rearrangements involving two breaks usually even- 

 tuate as either inversions or deletions (the symmetrical and asymmetrical 

 types diagramed in Fig. 9-3). Evidence from studies on Drosophila 

 suggests that deletions are produced about as frequently as inversions of 

 the same length (Fano, 1941). The frequency of X-chromosome inver- 

 sions used in this comparison was determined by salivary-gland-chromo- 

 some analysis; the frequency of X-chromosome deletions, which cannot 

 be ascertained accurately by that method, was determined by genetic 

 analysis of the exceptional females produced in a cross between an irradi- 

 ated wild-type male and a female carrying attached-X chromosomes with 

 recessive marking genes (Bishop, 1941). 



Other analyses of two-break rearrangements indicate that recombina- 

 tion within a chromosome limb is essentially at random. When inver- 

 sions were grouped in classes, according to length in number of divisions 

 within the euchromatic portions of the chromosomes (ranging from 0-18 

 divisions), the numbers corresponded closely with those expected on the 

 basis of random recombination (Bauer, Demerec, and Kaufmann, 1938; 

 Bauer, 1939b). A more intensive study of length of X-chromosome two- 

 break rearrangements, in terms of subdivisions (ranging from to 113 

 subdivisions, since each of the 19 divisions has 6 subdivisions), revealed 

 some departures from the values expected with random recombination 

 (Kaufmann, 1946a). Rearrangements measuring about 13 subdivisions 

 or multiples thereof were somewhat more frequent than those of other 

 lengths. This modal grouping suggests a pattern of coiling within the 

 X chromosome at the time of recombination that slightly increases the 

 opportunities for reunion of parts separated by the distance of the turn of 

 the coil, as compared with regions separated by shorter or longer 

 distances. 



No accurate measurements can be made of the length of rearrange- 

 ments having one break in the proximal heterochromatic region (as in 

 division 20 of the X) and one elsewhere along the chromosome (as in 



