CHROMOSOME ABERRATIONS IN ANIMALS 663 



from analysis of patterns of recombination, which suggest that the X 

 chromosome of the spermatozoon is coiled in a series of 13 or 14 gyres 

 (Kaufmann, 1946a)]. 



In a parallel study of the salivary-gland chromosomes of male larvae, 

 it was found that breakage occurs in the Y chromosome with a frequency 

 that is approximately equal to that in the limbs of the autosomes (Kauf- 

 mann and Demerec, 1937). Only 262 breaks were diagnosed in these 

 experiments, as compared with the 1606 indicated in Table 9-3, and there 

 was much greater variability in the distribution of breaks among the 

 chromosomes. Fewer breaks were detected in the Y chromosome than 

 would be expected on the basis of length in mitotic cells, but this was in 

 part due to inability to recognize in salivary-gland chromosomes the types 

 of rearrangements that are restricted to heterochromatin. Since the Y 

 chromosome is essentially heterochromatic, and is represented in the 

 salivary-gland nucleus by only a few discs, practically all inversions and 

 intercalary deficiencies escape detection. If the estimated number of 

 breaks involved in such rearrangements is added to the observed number, 

 the total for the Y chromosome approaches that expected on the basis of 

 mitotic chromosome length. These findings do not support the assump- 

 tion of Lea (1946) and Catcheside (1948) that greater sensitivity of the 

 X chromosome, as compared with the Y, accounts for the fact that in 

 some studies (e.g., Bauer, 1942; Catcheside and Lea, 1945a) the propor- 

 tion of female flies hatching from eggs fertilized by irradiated spermatozoa 

 was found to be lower than the proportion of male flies. Demerec and 

 Fano (1944), on the contrary, found only a slight effect of irradiation 

 on sex ratio, and questioned its significance. They suggested that loss, 

 as a result of a breakage-fusion-bridge cycle, may occur in either Y or 

 X chromosomes with approximately equal frequency, and that the num- 

 bers of dominant lethals contributed by these two chromosomes are 

 roughly comparable. 



The close correspondence between break frequency in the Y chromosome, 

 which is largely heterochromatic, and the autosomes, which are largely 

 euchromatic, suggests that breaks are distributed at random and in propor- 

 tion to length of the chromosome at the time of irradiation, regardless of 

 its euchromatic or heterochromatic properties. On the other hand, recent 

 studies, by genetic methods, of the frequencies of translocations induced 

 by X rays in D. virilis suggest that the Y chromosome participates in 

 exchange only about half as frequently as the various autosomes, although 

 it is of similar length (Baker, 1949). To what extent this result depends 

 on primary break production and to what extent on subsequent behavior 

 of breakage ends remains to be determined. Muller and his associates 

 also had decided, primarily on the basis of genetic analysis of X-chromo- 

 some inversions in D. melanog aster, that breaks in this chromosome are 

 not distributed at random. They suggested that breaks are produced at 



