CHROMOSOMAL MUTATIONS 



NORMAL 



Mil 





TRANSLOCATED 



a 



4 ::4 



Figure 80. Translocation Heterozygote. a and h diagrammatic, c actual chromo- 

 somes. (From Altenburg, "Genetics," 2nd Ed., Henry Holt & Co., Inc., 1957.) 



exchanged portions retain their original synaptic specificity, with the re- 

 sult that two different tetrads are bound together. 



Position Effects. In accordance with the classic gene theory (see the 

 preceding chapter), it might be expected that such chromosomal re- 

 arrangements would alter linkage relations, but would never actually 

 affect phenotypes. Yet in fact there are many examples in which each type 

 of chromosomal rearrangement behaves as though it were a gene muta- 

 tion: that is, it causes a definite phenotype, and is inherited according to 

 the usual principles of Mendelian heredity. Many are lethal in the homo- 

 zygous state, but some are not, and the fact that closely related species 

 can often be shown to differ by a few homozygous rearrangements shows 

 that these can play a definite role in evolution. An obvious explanation 

 of the mutational effect of rearrangements would be that the same force 

 which causes the rearrangement (breaking the chromosome) also causes 

 mutation of the gene nearest the break. Everyone who has worked in this 

 field agrees that most, if not all, of the mutational effects of chromosomal 

 rearrangements cannot be explained on this basis. But the final disproof 

 was obtained by Dubinin in the course of a study of a reciprocal trans- 

 location between the second and third chromosomes of Drosophila. The 

 untranslocated third chromosome carried the recessive gene hairy, which 

 causes excessive development of the bristles when homozygous. The trans- 

 located third chromosome, however, carried the normal allele of the hairy 

 gene. When these two genes were interchanged by a crossover, hairy 

 became dominant over normal. As crossing over has no such influence in 

 the absence of the translocation it must be that change of dominance is 

 a simple consequence of change of neighborhood: hairy gene in a normal 

 chromosome is recessive, while hairy gene in a translocated chromosome 

 is dominant. Because the rearrangement of the genes in the chromosome 

 seems to have a mutant effect independent of the genes, such chromo- 

 somal mutations are called position effects. 



The Bar Eye "Gene." The Bar Eye mutation of Drosophiki was the first 

 to be analyzed in terms of position effect. The eyes of normal flies are oval 

 shaped. The Bar gene, however, results in a smaller number of facets than 

 usual, and so the eyes are narrower than those of normal flies. Zeleny 

 showed that, in homozygous Bar-eyed stocks, about one fly in 1500 mutated 

 to the more extreme Double-Bar, while an equal number mutated back 

 to the wild type. By experiments involving crossing over between other 



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