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CHAPTER 14 



tion from unipolarity to bipolarity. Since 

 the chance of detecting and proving a change 

 from uni- to bipolarity is verj small indeed, 

 the occurrence of such a change cannot, at 

 present, be denied with certainty. Do muta- 

 tions to nonpolaritv occur'.' It is evident that 

 a unipolar or bipolar gene that mutates to a 

 Donpolar alternative must necessarily drop 

 out of the chromosomal line-up. If this hap- 

 pens, the freed, not-at-all-sticky gene will 

 not he linked to any chromosome. Since 

 no evidence has yet been presented for the 

 existence of genetic material liberated from 

 its chromosomal locus in this way, we cannot 

 give an affirmative answer at this point. 



The gene was first identified in sexually 

 reproducing individuals whose chromosomes 

 synapse during meiosis. Synapsis results 

 from the attraction between different seg- 

 ments of one or more chromosomes. That 

 different degrees of specific attraction exist 

 between genes is illustrated by the fact that 

 genes located in heterochromatin synapse 

 much less specifically than those found in 

 euchromatin. Specific genes (such as one 

 in maize called asynaptic) are known which 

 not only lack synaptic attraction for their 

 alleles but also destroy this attraction be- 

 tween pairs of genes at other loci, or cause 

 general desynapsis. The occurrence of col- 

 lochores — genes which assist in pairing — has 

 already been mentioned in Chapter 13. 

 Corresponding euchromatic loci located in 

 homologous chromosomes synapse with each 

 other whether or not the particular alleles 

 contained are identical or different. Yet 

 euchromatic genes in nonhomologs do not 

 usually synapse with each other, although it 

 is presumed that some presently nonallelic 

 genes were previously allelic. Consequently, 

 mutation must be capable of changing the 

 synaptic specificity of a gene; and it must 

 follow, at least in a general way, that iden- 

 tical genes attract each other more than non- 

 identical ones. 



Since at least some genes have multiple 



alleles, it is clear that different forms of a 

 gene do exist, and mutations of such genes 

 are not explicable merely in terms of their 

 complete loss or inactivation. Since some 

 mutations produce no visible change in the 

 handing pattern of salivary gland chromo- 

 somes of Drosophila, mutations involving 

 but a single gene, that is, gene mutations, 

 can be submicroscopic. At present, we can 

 only detect gene mutations by the pheno- 

 typic changes they produce. Consequently, 

 the characteristics of gene mutation must be 

 determined from the phenotypic changes 

 produced by recombinationally detected 

 genes. Accordingly, we are unable to de- 

 termine from such phenotypic changes 

 whether gene mutation involves the recom- 

 binational gene in toto, a one portion or site 

 within it. or many different sites within it. 

 If gene mutation involves a change in the 

 entire gene, then the material composition 

 of the genes detected by recombination and 

 by mutation would be identical. If, on the 

 other hand, the recombinationally detected 

 gene contains one or more sites at which 

 mutation can occur, the basic recombina- 

 tional unit of genetic material would be larger 

 than the basic mutational unit. Until such 

 time as critical evidence is obtained to the 

 contrary, we have no choice other than con- 

 tinuing to accept the mutational and recom- 

 binational genes as materially equivalent, an 

 assumption (Chapter 3, p. 36) which is in 

 accord with the law of parsimony. 



As mentioned in the first chapter, any 

 given gene is rather stable, having been faith- 

 fully replicated many thousands of times be- 

 fore a detectable mutation occurs. The 

 greater the sensitivity of our tests for de- 

 tecting mutations, however, the larger is the 

 frequency of mutation observed (recall the 

 detection of isoallelism, p. 59). It is rea- 

 sonable to assume therefore that transmis- 

 sible modifications of single genes do occur 

 which escape our present means of detection. 

 Nevertheless, within the limits of our present 



