BASES or CHANGIi 



The Gene as a Unit oj Cliaiigc 



How can wc find out something about what is inside that point, 

 the gene ? The evidence of the visible or breakable structure of the 

 chromosome, or of the linkage map inferred from the breeding 

 experiments, clearly no longer helps us. We have gone beyond the 

 limits of resolution by such means. Only by the study ot action 

 and of changes in action, that is of mutation, can we get any further 

 light on the matter. 



At most loci in the mapped chromosomes of maize or man, 

 Drosophila or Oenothera, we have discovered only two allelomorphs 

 of recognizably different effect. In addition to such simple allelo- 

 morphs, however, particular loci in scores of species show whole 

 series of alternatives, multiple allelomorphs as they are called. In 

 many such series the effect of the mutation of one allelomorph into 

 another has been seen in experiment. Bar eye is an extreme example, 

 where we have crossed the boundary into structural change. Most 

 of the differences, however, are intra-gcnic. 



Multiple allelomorphs are of particular importance when they 

 control the breeding systems of plants. One set or series then have 

 to be closely related in their actions, in order, as we shall see later, 

 to control the growth of the pollen down the style. This physiolog- 

 ical relationship is also seen in other series such as those at the white- 

 eye locus in Drosophila. Here, a number of allelomorphs produce 

 a range of intensity of pigmentation between none at all and the 

 full red of the dominant wild type. A dozen have been recorded and 

 there may well be more grades, and therefore more allelomorphs, 

 than have yet been separated by eye. Evidently all the allelomorphs 

 arc doing the same thing but to different degrees. One unit of action 

 therefore suffices to account for the range of behaviour and mutation 

 of this gene, although the range of its mutants shows the complexity 

 of its parts. 



Not all multiple allelomorphs, however, are so simple. A series 

 of four in Priinida sinensis includes three which affect only the size 

 of the greenish "eye" round the mouth of the corolla tube; the 

 fourth, which is recessive to all the rest, has the same effect on the 

 eye as the next most recessive but, in addition, shortens the style. 

 The series may be arranged, therefore, as in Table ii. 



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