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CHAPTER 23 



Because this chromosome fragmentation 

 takes place only in somatic cells, these 

 polarity changes can be ascribed to some 

 physiological difference between cells enter- 

 ing the somatic line and cells remaining in 

 the germ Hne. Should such changes be con- 

 sidered mutational? They should not, be- 

 cause the changes from bipolarity to uni- 

 polarity in Ascaris are numerous and simul- 

 taneous, and lack the novelty required of 

 mutations. 



It should be noted that while no unambigu- 

 ous case has ever been reported of a muta- 

 tion from unipolarity to bipolarity, the chance 

 of detecting and proving such a change is 

 very small indeed. Accordingly, the capacity 

 of mutation to change a gene in this manner 

 cannot, at present, be denied with any assur- 

 ance. Thus, although there are bipolar and 

 unipolar genes, a change in polarity via muta- 

 tion is clearly known to proceed only from 

 the former to the latter. Does this mean that 

 mutations to nonpolarity do not occur or 

 cannot occur? We cannot give a definite 

 answer at this point, since, thus far, we have 

 restricted our attention to genes all of which 

 have been localized in chromosomes. It 

 must be evident that a unipolar or bipolar 

 gene that mutates to a nonpolar alternative 

 must necessarily drop out of the chromosomal 

 Hne-up. If this happens, then the freed, not- 

 at-all-sticky gene will not be linked to any 

 chromosome. From what has already been 

 presented, there has been no evidence for the 

 existence of genetic material which has been 

 liberated from its chromosomal locus in this 

 way. Since the only kind of gene we have so 

 far identified is the chromosomally located 

 one, we must accept the simplest explana- 

 tion, and conclude that genes cannot occur 

 singly, two of them comprising the smallest 

 group possible. 



The gene was discovered by studying sexu- 

 ally reproducing individuals, in which many 

 of its recombinational properties were re- 

 vealed because of synapsis and the events 



consequent to this process. Synapsis is the 

 result of the attraction which exists between 

 corresponding segments of homologous chro- 

 mosomes. It is a remarkable fact that corre- 

 sponding loci located on homologous chro- 

 mosomes do attract each other in synapsis 

 even though the particular alleles contained 

 may be identical or somewhat different. Let 

 us assume that synapsis is directly dependent 

 upon the genie content of a chromosome. Is 

 this synaptic attraction between genes re- 

 stricted to alleles only? It is probable (see 

 Chapter 22) that what are nonallelic genes, at 

 present, were at some time, in the remote 

 past, allelic. Accordingly, mutation must be 

 capable of changing the synaptic specificity 

 of the gene, and it must follow, at least in a 

 general way, that identical genes attract 

 each other more than do nonidentical ones. 

 That different degrees of specific attraction 

 exist between genes is illustrated by the genes 

 located in heterochromatin (cf. p. 181), 

 which synapse much less specifically than 

 those found in euchromatin. Other genes 

 are known (for example, one in maize called 

 asynaptic) which either (1) not only lack 

 synaptic attraction for their alleles but also 

 destroy this attraction between pairs of genes 

 at other loci, or (2) cause general desynapsis. 

 Apparently, the gene is unrestricted muta- 

 tionally in the way it can affect synaptic 

 force. 



It should be clear from what has been 

 learned (in Chapter 22, for example) that 

 different genes do exist, mutations in them 

 not being explicable merely in terms of their 

 complete loss or inactivation, since each gene 

 can usually mutate to a number of different 

 allelic forms, which comprise a series of mul- 

 tiple alleles. Because different genes exist, 

 it is obvious that mutations occur involving 

 single genes. Since the gene is submicro- 

 scopic (a single band in the salivary gland 

 chromosome of Drosophila can contain more 

 than one gene), so also is a change produced 

 in a single gene. The only way we have of 



