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CHAPTER 24 



Strain with shorter, thinner bristles due to 

 the recessive allele bb {bobbed bristles). Now, 

 bb has a locus that happens to be present 

 both in the X and the Y chromosomes. You 

 might suppose that the male, or female, 

 homozygous for bb has bobbed bristles be- 

 cause this allele results in thinning and short- 

 ening the normal bristle. Since it is possible 

 to obtain otherwise diploid XYY males and 

 XXY females which carry three doses of bb, 

 one would expect, according to this view, that 

 the bristles formed would be still thinner and 

 shorter. But, on the contrary,- in the presence 

 of three doses of bb, the bristles are almost 

 normal in size and shape. This demonstrates 

 that bb functions in the same way as does 

 bb^, but to a lesser degree. Mutants having 

 a similar but lesser effect than the normal 

 gene are called hypomorphs. Many point 

 mutants are hypomorphs, since the addition 

 of further doses of them causes the pheno- 

 type to become more normal. 



Of the remainder of point mutants, most 

 are amorphs, producing no phenotypic ef- 

 fect, even when present in extra dose. White 

 eye {w) in Drosophila is an example of this. 

 Cases are also known of mutants which func- 

 tion as neomorphs; these produce a new 

 effect. Adding more doses of such a mutant 

 causes more departure from normal, while 

 adding more doses of the normal alternative 

 has no effect. 



We can represent the relationship between 

 the normal gene and its hypomorphic mutants 

 diagrammatically, as shown in Figure 24-4.^ 

 The vertical axis represents phenotypic ef- 

 fect, the normal effect being indicated by +. 

 The horizontal axis refers to the dosage of 

 the normal gene or of a hypomorphic mu- 

 tant. Notice that a single + gene by itself 

 almost produces the full normal phenotypic 

 effect (and often the difference between its 

 effect and the normal effect is not readily 

 detected). Two + genes reach the + pheno- 



^ As shown by C. Stern. 



3 Adapted from H. J. Muller. 



typic level. But, in the case of the hypo- 

 morphic mutant, even three doses may not 

 reach the phenotypic level produced by one 

 + gene (recall the discussion of bb). Note 

 also that genetic modifiers or environmental 

 factors, which would shift the position on the 

 horizontal axis and thereby shift the pheno- 

 typic effect, have less and less influence as one 

 proceeds from individuals carrying only one 

 dose of mutant toward individuals carrying 

 two + genes. You can see that natural selec- 

 tion would favor alleles resulting in pheno- 

 typic effects close to +, that is, near the 

 curve's plateau, for this would insure pheno- 

 typic stability. Any mutant which produced 

 such a phenotypic effect would, in the course 

 of time, become the normal gene in the pop- 

 ulation and would automatically be domi- 

 nant when heterozygous with a hypomorphic 

 gene alternative. This model illustrates how 

 the heterozygote with one + and one mutant 

 gene has practically the same effect as the 

 normal homozygote, and seems to be the best 

 explanation for most cases of complete or 

 almost complete dominance. This scheme 

 also illustrates why so few mutants are bene- 

 ficial, other things being equal, for the nor- 

 mal gene alternative already produces an 

 amount of phenotypic effect near optimum. 



In view of what was just discussed, you 

 can understand that hypomorphic and amor- 

 phic mutants are usually detrimental when 

 pure (homo- or hemizygous). But you may 

 wonder what their effects will be when 

 heterozygous with the normal gene. If they 

 are amorphs, the single + gene may fall 

 short of producing the full phenotypic 

 effect, and so such mutants would be ex- 

 pected to be slightly detrimental when het- 

 erozygous. Hypomorphs would be ex- 

 pected to be less detrimental or not at all 

 detrimental when heterozygous, at least with 

 respect to the trait for which they are classi- 

 fied as hypomorphic. But it should be re- 

 called that each gene affects many different 

 biochemical processes, and a mutant which 



