642 RADIATION BIOLOGY 



to be used, requires determination of crossover values between the 

 chromosomes involved in a translocation and their homologues. As indi- 

 cated previously, the apparent linkage between genes located in different 

 chromosomes in translocations is attributable to the inviability of the 

 recombination classes carrying duplications and deficiencies. This 

 apparent linkage can be utilized for determining genetically the loci at 

 which the chromosomes were broken and reunited. The method was 

 outlined by Dobzhansky (1936) in the review from which Fig. 9-9 is 

 reproduced. This represents a translocation between the second (black) 

 and third (stippled) chromosomes, both of which had been broken near 

 the middle. Females heterozygous for this translocation, which carried 

 in their normal homologues the series of genes indicated, were mated 

 with normal males that carried the same series of marking genes. Cross- 

 ing over took place in the female between the chromosomes involved in 

 the translocation and their normal homologues. The strongest linkage 

 was observed in this case between the genes cu and c, and between st and 

 pr, indicating that breakage and recombination had occurred between 

 these loci. The validity of this method of diagnosis has been confirmed 

 in several instances by parallel cytologic studies. 



2-2. TYPES OF INDUCED CHROMOSOMAL ABERRATIONS 



Any deviation from the standard pattern in number of chromosomes 

 or arrangement of their component units may be regarded as an aberra- 

 tion (Dobzhansky, 1936). Although changes conforming to these speci- 

 fications regularly occur under natural conditions, the frequency of their 

 occurrence can be increased enormously by ionizing radiations. The 

 types of alterations that have been induced in this manner include 

 duplications and deficiencies affecting individual chromosomes and sets 

 of chromosomes, and inter- and intrachromosomal rearrangements. 



2-2a. Haploids and Polyploids. Diploid individuals normally have 

 every chromosome represented in duplicate in cells that have not under- 

 gone meiosis. In such individuals occasional cells, or groups of cells, 

 may have three, four, or more sets of chromosomes, as a result of failure 

 of chromatids to separate after their multiplication by normal mitotic 

 processes or by endomitosis. Such polyploid cells can sometimes be 

 induced in a diploid organism by irradiation. Makino (1939) detected 

 tetraploid spermatocytes in testes of adults of Podisma mikado (Acrididae) 

 developing from irradiated nymphs. The observations, which were 

 restricted to three individuals, were not in themselves conclusive, since 

 polyploid cells occur in nature in the testes of some Orthoptera (Mickey, 

 1942; Ray Chaudhuri and Bose, 1948). However, Creighton and Evans 

 (1941) reported the formation in Chorthippus of giant spermatids by 

 direct transformation of primary spermatocytes whose normal course of 



