Mutable Genes 239 



the rose-a gene, this gene can also mutate to the dominant allele 

 during the formation of germ cells. Lavender-variegated plants 

 have been found with very large purple sectors sometimes 

 amounting, apparently, to half the plant. Such chimeras repre- 

 sent gene mutations during very early embryonic development 

 of the plant. Mutations at those stages are frequent in the 

 lavender line but are rare in the rose strains. The lavender gene 

 changes with high frequency very early and very late in ontog- 

 eny and is constant or mutates with a very low frequency at 

 other stages. The rose-a gene appears to mutate at a virtually 

 uniform rate throughout the whole life cycle. 



In Drosophila virilis, the recessive gene, miniature-3 produces 

 miniature-winged flies. As this gene is unstable, however, flies 

 with wild-type patches on an otherwise miniature wing have 

 been observed, and each patch indicates a somatic mutation 

 from the recessive to the dominant condition. Some flies from 

 miniature-3 parents have completely wild-type wings. In such 

 flies, the mutation from recessive to dominant occurred in the 

 formation of germ cells instead of in somatic tissue. 



Most of the mutations which occur in unstable genes are from 

 the recessive to the dominant and almost always are from the 

 mutant type to the wild type. Sometimes, however, the muta- 

 tion is not to the dominant but to a third allele, which is also 

 unstable. This condition is well illustrated by considering all the 

 recessive alleles at the miniature locus in Drosophila virilis. 

 There are five alleles at this locus other than the wild-type 

 alleles, and each produces a miniature wing of a different size. 

 Miniature-1, miniature-2, and miniature-4 are normal, stable 

 genes, mutating very infrequently, whereas miniature-3 and 

 miniature-5 are unstable. Furthermore, each of these two un- 

 stable genes exists in three forms. In each gene, the phenotypic 

 expression is the same, but the degree of stability is different. 

 The "alpha" or "a" form of each gene is unstable in both ger- 

 minal and somatic tissue; the "gamma" or "c" form is unstable 

 in somatic tissue; and the "beta" or "b" form does not revert 

 to the wild type in either kind of tissue. In these forms, muta- 

 tions occur not only to the wild-type allele but also to one of 

 the other forms of the same allele. This series suggests, as 

 Demerec has pointed out, that each allele at the miniature locus 



