1194 BIOLOGICAL EFFECTS OF RADIATION 



color mutations. In the first group (Patterson and Painter, 108; Patter- 

 son 105; perhaps also Gowen and Gay, 46, 47) the presence of patches in 

 the eye seems to be due to occasional losses of fragments of chromosomes 

 in the cell divisions. For instance, in the "mutable translocation" 

 described by Patterson (105) a section of the X-chromosome carrying the 

 wild-type allelomorph of white is transposed onto the fourth chromosome. 

 The union between this section and the fourth chromosome is, however, 

 so weak that the former frequently drops off and is lost in the cytoplasm. 

 If a female carrying this translocation is made heterozygous for the gene 

 white, the cells in which the fragments are lost are white, hence the 

 mosaicism in the eye arises. 



The second group of the dominant eye colors, which is most interesting 

 for the purposes of the present discussion, seems not to be connected with 

 losses of fragments of chromosomes. The mutations belonging to this 

 group produce eyes of a color different from the wild type, with or without 

 patches of still differently colored tissue. Muller, Van Atta, and Glass 

 (1. c.) described a series of mutations called "Plum" (or "dilute"). They 

 are all allelomorphic to each other, and also to the second chromosome 

 gene brown, which sometimes mutates spontaneously producing recessive 

 eye-color changes of a more or less extreme type. In every known 

 instance a mutation from w ild type to Plum is correlated with a breakage 

 of the second chromosome in the neighborhood of the brown locus. In 

 at least two of the Plum allelomorphs the second chromosome carries an 

 inversion one end of which is at brown, and the other lies in the vicinity 

 of the spindle fiber, close to the locus of the gene light {It, cf. Figs. 4 

 and 5). Schultz and Dobzhansky (116) have found that in these two 

 cases Plum is allelomorphic to both brown and light. Thus, mutations 

 have taken place at the loci of both breakages producing the inversion. 

 It seems probable that every time such an inversion arises, mutations 

 take place at both the brown and the light loci. According to Glass 

 (43, 44), another dominant eye color. Grape, is allelomorphic to the third- 

 chromosome recessive gene peach, and grape is associated with a trans- 

 location involving a breakage of the third .chromosome at peach. 



The gene Bar in Drosophila melanogaster arose by a spontaneous 

 mutation from wild type. This mutation has been observed only once. 

 Mutations at this locus, consequently, must be very rare. Dobzhansky 

 (30) found a translocation in which the Z-chromosome was broken at 

 Bar, and simultaneously with the appearance of the translocation a 

 mutation to a reces.sive allelomorph of Bar, called baroid, took place. 

 This case is remarkable because Sturtevant (130, 132) has proven that 

 there is no wild-type allelomorph of Bar in wild-type Drosophila. Conse- 

 quently, the mutation to baroid can not be interpreted as a loss or injury 

 of the wild-type allelomorph of a gene located close to the locus of 

 breakage. 



