198 



CHAPTER 14 



zygous-Bar F2 females, further study of the 

 sive lethal is possible in F L . and subse- 

 quent generations. Such studies reveal that 

 certain lethals are associated with intergenic 

 changes; Lethals not associated with inter- 

 genic changes are designated as recessive 

 lethal point mutants. The Base technique 

 can also be used to deteet recessive lethals 

 that occur In a P, Base chromosome, the 

 absence of Base males among the F 2 prog- 

 eny indicating such a mutation. Moreover, 

 if the environmental conditions are standard- 

 ized, it becomes possible to detect hemi- 

 zygous mutants which either lower the via- 

 bility of the F2 males without being lethal 

 or raise their viability above normal. The 

 opportunity for studying the viability effects 

 of recessive lethals in heterozygous condition 

 is also provided by this technique. 



Although the Base technique can also be 

 used to detect X-linked mutants producing 

 a visible morphological change when hemi- 

 zygous, all those "visibles" which are also 

 hemizygous lethals are missed. The "Maxy" 

 technique 5 overcomes this difficulty. In this 

 method, the tested female has fifteen X- 

 linked recessive point mutants on one homo- 

 log and their normal alleles on the other. 



-See H. J. Muller (1954). 



Suitable paracentric inversions maintain the 

 identity oi these chromosomes in successive 

 generations. Mutants are detected when 

 such females show one or more of the re- 

 cessive traits. Maxy detects, therefore, any 

 mutation involving the normal alleles of the 

 fifteen reccssives, provided that the mutant 

 does not produce the normal phenotype 

 when heterozygous with the recessive allele 

 and is not a dominant lethal. Once such 

 mutants are obtained, they can be screened 

 for point mutants. 



The study of recessive lethals in the X 

 chromosome and in the autosomes shows 

 that there are hundreds of loci whose point 

 mutations may be recessively lethal. It 

 should be noted that the recessive lethals 

 detected by Base and the visibles detected 

 by Maxy are not mutually exclusive types of 

 mutants, for some Maxy-detected visibles 

 are lethal when hemizygous, and about ten 

 per cent of Base-detected hemizygous lethals 

 show some morphological effect when hetero- 

 zygous. It can be stated, in general, that 

 any mutant in homo- or hemizygous condi- 

 tion which is a "visible" will produce some 

 change in viability, and, conversely, that any 

 mutant which affects viability will produce a 

 "visible" effect, "visible" at least at the bio- 

 chemical level. 



SUMMARY AND CONCLUSIONS 



The mutational units of a genotype are, in order of size: the genome; the chromosome; 

 chromosomal segments involving more than one gene; and the gene. Since a recom- 

 hinational gene can have multiple alleles, gene mutation may involve the entire recom- 

 hinational unit or one or more mutational sites within it. Although the genes delimited 

 operationally by recombination and by mutation may not be materially equivalent, we 

 shall continue to assume that this is so until we have evidence to the contrary. 



The occurrence of gene mutation is not limited by any ploidy, type of cell or gene, or 

 effect it can have on synapsis. It is limited with respect to the effect it can have on 

 gene polarity. Tripolar genes are excluded, bipolarity being the usual and unipolarity 

 the less usual alternative. 



Point mutations are the remainder of all mutations not identifiable as intergenic 

 changes. Since point mutants include gene mutants, the former can be studied to 

 reveal the mutational characteristics of the gene. The frequency of point mutants 

 increases linearly with the dose of high-energy radiations; there is no effect from dose 



