194 



CHAPTER 14 



alternatives, only the more advantageous 



alleles were retained, and these are the ones 

 found in present populations. So. a point 

 mutation today is likely to produce one of 

 the genetic alternatives which occurred also 

 in the past but had been eliminated beeause 

 ol its lower biological fitness, that is, its 

 lower reproductive potential. It should be 

 realized, moreover, that reproductive poten- 

 tial is the result of coordinated action of the 

 whole genotype. The genotype may be 

 likened to the machinery that makes modern 

 automobiles — the automobile representing 

 the phenotype — with the environment fur- 

 nishing the necessary raw materials. Pres- 

 ent genotypes, like the machines that manu- 

 facture automobiles, are complex and have 

 had a long evolutionary development. The 

 chance that a newly-occurring point muta- 

 tion will increase reproductive potential is 

 just as small as the chance that a random 

 local change in the present machinery will 

 result in a better automobile. 



The differences between the phenotypic 

 effect of a point mutant and its normal alter- 

 native can be studied by adding more repre- 

 sentatives of the mutant allele to the geno- 

 type and examining the effect. In Drosoph- 

 ila, for example, the normal fly has long 

 bristles when the normal, dominant gene 

 bb + is present. A mutant strain has shorter, 

 thinner bristles because of the recessive allele 

 bb (bobbed bristles), which — it should be 

 recalled — has a locus both in the X and the 

 Y chromosomes. We might suppose that 

 the male, or female, homozygous for bb has 

 bobbed bristles because this allele results in 

 thinning and shortening the normal bristle. 

 Since otherwise-diploid XYY males and 

 XXY females can be obtained which carry 

 three bb alleles, according to this view, one 

 would expect the bristles formed to be even 

 thinner and shorter than they are in ordinarv 

 mutant homozygotes. But, on the contrary, 1 



3M 1 • 

 DOSAGE OF GENES 



figure 14-1. The relationship between dos- 

 age of normal and mutant genes and their 

 phenotypic effect. 



in the presence of three representatives of 

 bb — that is, three doses of bb — the bristles 

 are almost normal in size and shape. This 

 finding demonstrates that bb functions in 

 the same way as bb+ does, but to a lesser 

 degree. Mutants whose effect is similar but 

 less than the normal gene's effect are called 

 hypomorphs. Many point mutants are hy- 

 pomorphs, since, in the absence of the nor- 

 mal gene, additional doses cause the pheno- 

 type to become more normal. 



Of the remaining point mutants, most are 

 amorphs; these produce no phenotypic effect 

 even when present in extra dose. One ex- 

 ample is the gene for white eye (w) in Dro- 

 sophila. 



Some mutants, neomorphs, produce a new 

 effect — adding more doses of a neomorphic 

 mutant causes more departure from normal, 

 whereas adding more doses of the normal 

 alternative has no effect. 



The relationship between the normal, 

 wild-type gene and its hypomorphic mutants 

 is indicated diagrammatically in Figure 

 14-1. 1 ' The vertical axis represents pheno- 

 typic effect; the normal, wild-type effect is 



1 As shown by C. Stern. 



-Adapted from H. J. Muller. 



