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CHAPTER 45 



ies, we could determine more about the 

 precise nature of the nuclear gene change. 

 As you may have surmised from the previous 

 work discussed, the vast majority of gene- 

 based traits that have been carefully analyzed 

 are determined by genes contained in chromo- 

 somes. Thus, we would usually fail to find 

 the extranuclear gene we started out to de- 

 tect.) 



But consider the genetic alternatives for 

 resistance and susceptibility to CO 2 gas. One 

 strain of Drosophila flies can be exposed to 

 pure CO2 for as long as 15 minutes and 

 recover without apparent eff"ect, while flies of 

 another strain so exposed are almost invari- 

 ably killed. Using marked chromosomes, it 

 is found that C02-sensitivity is not linked to 

 any chromosome of the normal genome. In 

 fact, it is possible, by appropriate crosses, to 

 replace each of the chromosomes present in 

 the sensitive strain by a corresponding chro- 

 mosome of the resistant strain. Yet, after 

 this is done, the flies produced are still 

 sensitive to COo! It is still possible, however, 

 that the sensitive strain has somehow ob- 

 tained an additional nuclear chromosome 

 which, of course, would not be linked to any 

 of the usual ones. Since the C02-sensitivity 

 trait does not segregate in the progeny of 

 hybrids between sensitive and resistant lines, 

 this must mean that such a supernumerary 

 chromosome cannot occur singly (in the 

 individual that is hybrid for sensitivity) or as 

 a pair (in flies of the pure sensitive strain). 

 Moreover, cytological examination reveals 

 no additional nuclear chromosome. Even so, 

 the latter finding is not a conclusive argument 

 against a nuclear locus for C02-sensitivity, 

 since "chromosomes" so small that they es- 

 cape cytological detection are known to exist 

 from recombinational evidence. 



It is found, however, that while the sensi- 

 tive female regularly transmits C02-sensi- 

 tivity to some progeny, the sensitive male 

 does so only under special circumstances. It 

 is still possible to conceive that a nuclear gene 



for sensitivity might somehow be preferen- 

 tially excluded from a nucleus destined for a 

 sperm but not from one destined for an 

 egg. It seems much more reasonable, how- 

 ever, to attribute the nontransmission of CO2- 

 sensitivity through the sperm as being the 

 consequence of the relatively minute amount 

 of cytoplasm included in a sperm, as com- 

 pared with the amount present in an egg of 

 Drosophila. It is, therefore, very probable 

 that C02-sensitivity is due to the presence of 

 a body, called sigma, which is mutable and 

 proves to have many of the characteristics of 

 a virus, including infectivity by experimental 

 means. ^ Sigma is not visible within the cell, 

 and so we do not know more about its 

 location. 



Consider next another case, in Drosophila, 

 already mentioned on p. 107. In this case, 

 females mated to normal males give rise 

 almost entirely to females. This trait has a 

 genetic basis and also is not transmitted by 

 males. Moreover, it is infective and un- 

 hnked to the usual chromosomes. It turns 

 out that this female-producing-female trait 

 is intimately associated with the presence of 

 a spirochaete which can be seen in the blood. 

 We might mention also the occurrence in 

 corn of a phenotypic change which is associ- 

 ated with the presence of readily detectable, 

 supernumerary, heterochromatic (so-called 

 B) chromosomes. 



None of the three cases just described con- 

 clusively demonstrates the existence of intra- 

 cellular but extranuclear genes. However, 

 they serve to illustrate the types of outcome 

 which may be obtained, when investigations 

 to prove the existence of such genes start 

 with a study of genetic recombination. In 

 view of such results, you can readily appreci- 

 ate the advantage of correlating potentially 

 extranuclear genes directly with objects 

 observable in the cytoplasm. This advantage 

 is held in the case of particles called kappa, 



^ Sigma has been studied mostly by P. L'Heritier, 

 G. Teissier, and coworkers. 



