640 RADIATION BIOLOGY 



second and third chromosomes (represented by the symbols +; + ) are 

 mated with females whose second chromosomes carry the dominant 

 markers Curly (Cy) and Plum (Pm), and the third chromosomes the 

 dominants Hairless (H) and Stubble (Sb). The heterozygous Fi flies, 

 which are of four types with respect to the dominant marking genes — 

 namely, Cy;H, Cy;Sb, Pm;H, and Pm;Sb — are mated individually with 

 unrelated wild-type flies of the opposite sex. Figure 9-8 represents a 

 cross between heterozygous Fi males and wild-type females. The pres- 

 ence or absence of translocations between the second and third chromo- 

 somes is determined by examination of the F2 cultures. If no transloca- 

 tion has been induced by irradiation, each of the second and third 

 chromosomes of the Fi males, whether paternal or maternal in origin, 

 will carry a normal complement of genes. Independent assortment at 

 meiosis will yield four types of spermatozoa, with respect to the mutants 

 under consideration, which will produce, by fertilization of eggs bearing 

 the wild-type chromosomes, four kinds of F 2 progeny in approximately 

 equal numbers (for example, from the cross Cy ;H d" by + ; + 9 Cy;H, 

 Cy;-\-, -\~',H, and + ;+ males and females). If, on the other hand, an 

 induced translocation is present, only the maternally derived second and 

 third chromosomes will carry an unaltered complement of genes, since 

 those of paternal origin will have exchanged parts with each other. 

 Independent assortment at meiosis will yield four types of spermatozoa, 

 but two of them will carry some genes in duplicate and be deficient for 

 others (bottom row of Fig. 9-8). Eggs fertilized by such spermatozoa 

 will not as a rule give rise to viable progeny, although a duplication or 

 deficiency zygote may occasionally survive to produce an individual 

 possessing special somatic characteristics. Eggs fertilized by the other 

 two types of spermatozoa (those carrying the second and third chromo- 

 somes with the dominant markers, and those carrying the two chromo- 

 somes that have exchanged parts), both of which transmit a complete 

 set of genes, will produce viable progeny. The occurrence, in the cross 

 illustrated, of only two classes of F 2 progeny — namely, Cy;H and + ;H — 

 will thus serve as an index to the induction of a reciprocal translocation. 



Translocations may be detected in a similar manner by irradiating 

 males whose chromosomes carry dominant marking genes, and mating 

 them with nonirradiated wild-type females. This procedure is described 

 in detail by Dobzhansky (1936). 



Another technique for the detection of translocations involving a par- 

 ticular chromosome is based on phenotypic modification accompanying 

 change in position of a specific gene. Thus the cubitus interruptus (ci) 

 position effect in D. melanogaster, which alters the normal pattern of wing 

 venation, is caused by a translocation involving the fourth chromosome, 

 whereby the dominance of the wild-type allele of cubitus interruptus is 

 weakened (Dubinin and Sidorow, 1934). Using this criterion of assay, 



