211 



3M 1 

 DOSAGE OF GENES 



2 + 



FIGURE 24-4. The relationship between dosage of normal and mutant genes 

 and their phenotypic ejfect. 



is hypomorphic with regard to one trait may 

 be amorphic with respect to another one. 

 Thus, in Drosophila, the normal allele «/»/•+ 

 results in dull-red eye color and also pigments 

 the Malpighian tubules. One of its alleles, 

 apr, results in a lighter eye color (being 

 hypomorphic in this respect) but no color in 

 the Malpighian tubules (being amorphic in 

 this respect). 



Experience confirms expectation in this re- 

 gard. It has been shown that most "reces- 

 sive" lethal point mutants result in some 

 detrimental effect on reproductive potential 



when heterozygous. Such mutants are not 

 completely recessive, therefore, and it has 

 been found, in Drosophila, that when hetero- 

 zygous, they are cause of death before 

 adulthood of about four per cent of indi- 

 viduals. Mutants, which are detrimental 

 but not lethal when pure, also usually show 

 such a detrimental effect when heterozygous, 

 this effect being somewhat less than that 

 produced by heterozygous lethal point mu- 

 tants. The principles of phenotypic action 

 discussed here apply both to spontaneous 

 and to induced point mutants. 



SUMMARY AND CONCLUSIONS 



The details of the Base and Maxy genetic schemes for the detection of recessive lethal and 

 recessive visible mutants in Drosophila are described. Tiie study of point mutants of these 

 types and others reveals that almost all are detrimental to the reproductive potential of 

 individuals when pure (not hybrid ), and to a lesser extent when hybrid. Accordingly, most 

 point mutants are not completely recessive to their normal genetic alternatives. Most 

 normal genes fail to produce the full normal phenotypic effect in single dose, and most point 

 mutants act on the phenotype in a hypomorphic or amorphic manner. 



