MUTATION 253 



type if crossed to a or to &. Similarly for a mutation in 

 the fourth and in the fifth normal gene. Now this is 

 exactly what does not take place when members of an 

 allelomorphic series are crossed — they do not give the 

 wild type, but one of the other mutant types or an inter- 

 mediate character. Evidently independent mutation in a 

 nest of linked normal genes will not explain the results 

 if the new genes arise directly each from a different nor- 

 mal allelomorph. 



But suppose, as shown in Fig. 4 (line C) after a muta- 

 tion had occurred in the first gene a new mutant, h, arose 

 from a new gene, and from h a mutation arose in a third 

 gene c, and c similarly gave rise to d; then a crossed to b 

 will give a (or something intermediate if the heterozygote 

 is an intermediate type). Likewise c crossed to b will 

 give b, or c crossed to a will give a, etc. If mutant allelo- 

 morphic genes in a series such as C, a, b, c, d, e, arise as 

 successive steps, i.e., Ca to Cb and Cb to Cc, etc., then 

 the hypothesis of closely linked genes would seem to be a 

 possible interpretation of the data, but if they do not 

 arise in this way, but by independent mutations from the 

 wild type (or even from each other, but not seriatim), then 

 they must be due to mutations in the same gene : for, to 

 assume that they are not, requires that, when the second 

 mutation took place both gene a and gene b mutated at 

 the same time, and that when c appeared three genes 

 mutated, when gene d appeared four; when gene e five 

 genes mutated at once, four of them being mutant genes 

 that have already arisen independently. Such an inter- 

 pretation is excluded, since it is inconceivable, even in a 

 readily mutating form like Drosophila, that five muta- 

 tions could have occurred at the same time in distinct but 

 neighboring loci. As has been stated, the evidence from 

 Drosophila shows positively that multiple allelomorphs 

 arise at random. 



Only two members of a series of multiple allelomorphs 

 can be present in any one individual, and in the case of 



