INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1175 



involved in the translocation and their normal homologues. Figure 2 

 represents a translocation in which both the second (black) and the third 

 chromosomes (stippled) are broken at the middle, and halves of these 

 chromosomes are exchanged. It is obvious that in such a case the 

 strongest linkage would be observed between the genes cm and c, and 

 between st and pr, indicating that breakages have taken place between 

 these genes. By this method the loci at which chromosomes have been 

 broken in translocations may be determined and entered on the genetic 

 maps of these chromosomes. Cytological investigation of individuals 

 carrying translocations gives independent evidence for the correctness 

 of the interpretation reached on the basis of genetic studies. 



As shown in the foregoing, deficiencies and duplications, provided 

 they are viable, can be obtained in the offspring of translocations. 

 Painter and Muller (98) proposed a convenient direct method for the 

 detection of duplications for sections of the A'^-chromosome of Drosophila 

 melanog aster. Wild-type males are treated with X-rays and crossed to 

 females having attached X-chromosomes, and homozygous for a desired 

 series of sex-linked recessives. X-ray treatment causes fragmentation 

 of the X-chromosome in some of the spermatozoa of the treated males. 

 Some of the fragments are lost. Such spermatozoa may fertilize eggs 

 carrying the two attached X's (the attached-X females produce two 

 kinds of eggs: those carrying two attached X-chromosomes, and those 

 carrying a F-chromosome, L. V. Morgan, 69). The result is the produc- 

 tion of zygotes having two complete X's (coming from the mother), 

 and a fragment, a duplication, of a third X (coming from the father). 

 Such zygotes can be recognized by the suppression of some of the reces- 

 sives located in the attached X's by the dominant allelomorphs located 

 in the duplication. 



In an experiment of the writer, wild-type treated males were crossed 

 to attached-X females homozygous for the sex-linked recessives y, W, ec, 

 and /. The following offspring were obtained : 



y w ecf 9 

 Wild-type cf 

 Superfemales 

 Wild-type 9 

 y W ecfd' 



Duplication 

 Duplication 

 Duplication 

 Duplication 

 Duplication 



iV^ ecf 9 

 ecf 9 

 f 9 



Wild-type 9 

 y W ec 9 



8 

 9 

 6 

 1 

 1 



The normal offspring are y w" ec f females and wild-type males; wild- 

 type females and y w" ec/ males are due to the occasional detachment of 

 the X-chromosomes in the mother. The w"' ec/ females carry duplicating 

 fragments containing the locus of y; the ec f females — a duplication for 

 y and w"' ; the / females — a duplication for y, w", and ec ; the y W ec females 

 —a duplication for /. It is interesting to note that the male offspring 

 of the original duplication females crossed to any male are invariably 



