Position Effect and Allelism in Drosophila 



195 



quences and produce position effects. And, 

 in fact, position effect is known to occur in 

 Oenothera. 



Finally, let us consider the morphology 

 of chromosome regions found to contain 

 pseudoalleles. The white series is associated 

 with a double band {doublet) in the salivary 

 gland chromosome; apr may be in one band, 

 w in the other. The vermilion series is asso- 

 ciated with a doublet on the X chromosome, 

 while the bithorax series (composed of five 

 separate pseudoallelic loci) is connected with 

 two doublets. (This indicates, what is proved 

 by other data, that a band may contain 

 more than a single gene.) Since there are 

 many doublets in salivary chromosomes, this 

 suggests that genes located in these regions 

 will be found to be pseudoallelic. 



How can we explain the origin of appar- 

 ently adjacent loci with similar types of 



action? There are several possible ways in 

 which this situation may have come about. 

 One explanation is that during the course of 

 evolution, adjacent genes, having different 

 effects, mutated to alleles which performed 

 similar functions, and therefore were ad- 

 vantageous. A second explanation invokes 

 a selective advantage of rearrangements 

 which brought together widely separated non- 

 alleles with similar functions. While both of 

 these mechanisms may explain some of the 

 cases found, it seems more likely that most 

 adjacent and similar genes arose as duplica- 

 tions that occurred once, or more times (as in 

 the bithorax case), in the ways already de- 

 scribed in Chapter 19. Following duplica- 

 tion, the linear array of originally identical 

 genes would become somewhat different 

 from each other functionally because of mu- 

 tation, thereby fostering evolution. 



SUMMARY AND CONCLUSIONS 



The same genie matter arranged in different ways may have different phenotypic conse- 

 quences. The shuffling of genes which produces position effects may be accomplished by 

 means of structural changes in chromosomes and by means of crossing over preceded either 

 by aneusynapsis or eusynapsis. While position effect may be one of the consequences of 

 mutation, it itself does not represent mutation. 



While the decision is usually valid that two genes located in totally different parts of the 

 genome are nonallelic whether or not they have similar or different phenotypic effects (on 

 the presumption that one is not an allele which was transported to a new location via struc- 

 tural change), statements made in earlier Chapters that two genes are allelic may often have 

 been invalid. To properly apply the term allele to two similarly acting genes, apparently 

 at the same map locus, it is necessary to perform exhaustive tests to recombine them via 

 crossing over. Recombination is demonstrated by the detection of a cis-trans position 

 effect, which proves the genes to be pseudoallelic and, therefore, nonallelic. Failure to 

 obtain a cis-trans position effect after exhaustive trials is taken to mean the genes involved 

 are alleles, though this necessarily includes possible cases of separable nonalleles which fail 

 to give this effect. 



Linear nests of genes with similar effects have probably arisen by one or more duplications 

 in situ of an ancestral gene, followed by mutations that led to differentiation in their effects. 



Position effect is attributed to some dependency which exists between the gene products 

 of adjacent nonalleles. 



REFERENCES 



Bridges, C. B., "The Bar 'Gene' a Duplication," Science, 83:210-211, 1936. Reprinted in 

 Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 

 1959, pp. 163-166. 



