CHROMOSOME ABERRATIONS IN ANIMALS 



641 



Eberhardt (1939) irradiated flies carrying a normal fourth chromosome 

 and determined the frequency of translocations from the proportion of 

 progeny showing interruptions in the cubital vein. 



In a more elaborate experiment designed to disclose exchanges among 

 all four chromosomes of an irradiated spermatozoon, Patterson, Stone, 

 Bedichek, and Suche (1934) mated irradiated wild-type males with 

 females having attached-X chromosomes homozygous for the mutant 

 gene yellow (yy), the second chromosome homozygous for brown (bw), 

 the third for ebony (e), and the fourth for eyeless (ey). The hetero- 

 zygous F! males were backcrossed individually to yy; bw, e; ey females. 



Fig. 9-9. Diagram illustrating the genetic technique for determining points of ex- 

 change between chromosomes in Drosophila melanogaster. A translocation-carrying 

 female heterozygous for a series of genes (left) is crossed to a male free from the 

 translocation and homozygous for the same genes (right). (From Dobzhansky, in 

 Duggar's "Biological Effects of Radiation," 1936.) 



Since these males carried X chromosomes received from their irradiated 

 fathers, many different types of reassociation could be detected by 

 examination of F 2 cultures, e.g., X;2, X;3, X;4, 2;3, 2;4, 3;4, X;2;3, 

 X ;2 ;4, X ;3 ;4, 2 ;3 ;4, and X ;2 ;3 ;4. Some duplication and deficiency types 

 were also viable and could be detected by criteria that will be indicated 

 presently. Translocations involving the Y chromosome were not 

 detected in these experiments because the females that came from eggs 

 fertilized by Y-bearing spermatozoa were not tested. 



When translocations have been diagnosed by such genetic methods, 

 they can usually be perpetuated in cultures, and the positions of the 

 breaks involved in the rearrangement can subsequently be determined by 

 either of tw r o procedures. The most informative and least laborious is 

 examination of salivary glands of individuals carrying the translocation 

 in heterozygous condition. The alternative method, which was the first 



