150 



( II M'TI R 1 1 



not be due to the rare combination of two 

 already-existing unlinked nonalleles since, at 

 most, onlj one quarter of the progenj would 

 have the novel phenotype. Consequently, 

 neither segregation nor independent segrega- 

 tion could be associated with ttie appearance 

 or nonappearance o\ Hexaptera. The new 

 phenotype might have appeared alter a rare 

 crossing over between two \er\ close loci 

 brought two previously separated nonalleles 

 into the same chromosome. Once produced, 

 this new combination of linked genes would 

 almost always remain intact and be trans- 

 mitted to one half of the progeny. How- 

 ex er. suppose also that the parents' chromo- 

 somes were suitably marked witli genes, and 

 it was found that the chromosome region. 

 essential for the production of the new phe- 



I IGl Rl 11-1. 

 melanogaster. 

 1949.) 



The Hexaptera phenotype in D. 

 (from Genetics, vol. 34, p. 13, 



notype, was o\' a noncrossovcr type. In 

 such a case, crossing over would not explain 

 the results either. 



The only reasonable remaining explana- 

 tion would be that a novel change, a muta- 

 tion, occurred in the genetic material. We 

 sec. therefore, that when the mutant pro- 

 duces a dominant phenotypic effect it is not 

 too difficult to determine whether a novel 

 phenotype is due to mutation rather than to 

 genetic recombination. 



In the case of a dominant mutant, only 

 one parent needs to have a specific genotype 

 to produce a dominant mutant trait in the 

 progeny; no particular genetic recombina- 

 tion is a prerequisite. In other cases, the 

 novel phenotype appears in the progeny only 

 when both parents have specific genotypes, 

 and genetic recombination is required for 

 its appearance. Note that the detection of 

 a completely recessive autosomal mutant 

 gene is postponed for the number of genera- 

 tions required for two heterozygotes to mate 

 and produce a mutant homozygote. Before 

 a recessive mutant becomes homozygous, 

 many generations may elapse, during which 

 time the mutant allele may become relatively 

 widespread in the population in heterozygous 

 condition. When the genotype of the pop- 

 ulation is uniform or is known, it may be 

 possible to trace back to the origin of a new 

 recessive mutant. If, however, the popula- 

 tion genotype is not known, it is impossible 

 to determine when a recessive mutant first 

 arose, and it may be considered — correctly 

 or not — one of the genes normally present 

 in the population. 



Obviously, the detection of mutants, both 

 recessive and dominant, would be made rela- 

 tively easy by using pure lines. As men- 

 tioned in Chapter 1. suddenly-appearing 

 phenotypic variants which are due to muta- 

 tion and not to environmental fluctuation are 

 occasionally found in pure lines of self- 

 fertilizing bean plants. When completely 



