138 



CHAPTER 18 



FIGURE 18-1. The Hexapteni p/ienolype in D. 

 melanogaster. {By permission of Genetics, Inc., 

 vol. 34, p. 13, 1949.) 



for Hexaptera. However, such a new pheno- 

 type could have arisen by genetic recombina- 

 tion, due to the occurrence of a crossover 

 chromosome, which was very rare because 

 the nonallelic genes involved were extremely 

 close together. Once produced, this combi- 

 nation of linked genes would remain intact 

 and be transmitted to one half of the progeny. 

 But suppose also that the chromosomes of 

 the parents of the first Hexaptera were suit- 

 ably marked with genes, and it was found that 

 the chromosome region whose presence is 

 essential for the production of the new pheno- 

 type was of a noncrossover type. Then 

 crossing over would not explain the results. 

 The only reasonable explanation remaining 

 would be that a novel change had occurred 

 in the genetic material, a mutation, which 

 produced a dominant phenotypic effect. You 

 see, then, that under certain circumstances it 



may be possible, without great difficulty, to 

 identify a novel phehotype as being due to 

 mutation rather than to genetic recombina- 

 tion, when the mutant produces a dominant 

 phenotypic effect. 



When, however, the novel phenotype re- 

 quires the mating of two particular individ- 

 uals for its appearance in progeny, it is much 

 more difficult to decide whether the genetic 

 recombination required for its appearance 

 involves old genes or a recently arisen mutant 

 which is apparently completely recessive. 

 Note, after the mutational origin of a com- 

 pletely recessive autosomal gene, that its 

 detection is postponed for that number of 

 generations which is required for two hetero- 

 zygotes to mate and produce a mutant homo- 

 zygote. Under certain conditions, many 

 generations may elapse before the recessive 

 mutant becomes homozygous, during which 

 period the mutant allele may become rela- 

 tively widespread in the population in hetero- 

 zygous condition. In this event, one cannot 

 decide when the mutant first arose, and it 

 may be considered part of the old pool of 

 genes present in the population. It would be 

 easier to identify a phenotype as the result of 

 a recessive mutation if the genotype of the 

 population was known to be uniform prior 

 to this. You see, therefore, why the detection 

 of mutants, both of recessive and of dominant 

 types, is made relatively easy by the employ- 

 ment of pure lines. The pure line procedure 

 for detecting mutants was used with self- 

 fertilizing beans, as described in Chapter 1. 

 As mentioned there, sudden phenotypic 

 changes, not due to environmental fluctua- 

 tion, are occasionally found which clearly 

 represent mutations, not recombinations from 

 genotypically different parents. In cases 

 where completely pure lines cannot be ob- 

 tained because self-fertilization does not 

 occur, detection of mutations is facilitated 

 when they involve genes for which both 

 parents are homozygous or completely known 

 genotypically. 



