Mutable Genes 237 



Mutable Genes 



In 1914, Emerson suggested that some genes might not be 

 completely stable. He studied a variegated variety of maize 

 that had a white pericarp with numerous red spots of varying 

 size. Genetically, these plants were homozygous for the recessive 

 gene for white. Emerson considered that this gene must be 

 unstable, that it can mutate spontaneously into the dominant 

 allele for red, and that each red spot on the kernel is made 

 up of cells which came from one cell in which such a mutation 

 arose. This was a very revolutionary idea at that time. More 

 recently, Jones has found other spotted kernels in maize which 

 he believes are caused by somatic gene mutations of a similar 

 nature. In Jones's material, six colored spots that were found 

 on otherwise colorless seeds were composed of six to forty-three 

 colored aleurone cells. These cells were similar in size, shape, 

 and color to the aleurone cells of a colored variety of the geno- 

 type AC R Pr, although the particular strain was homozygous 

 for c. The suggestion was made that gene c has mutated to C 

 upon these six occasions and that each colored spot represents 

 one somatic mutation from the recessive to the dominant allele. 



Since Emerson's discovery of mutable genes, several others 

 have been found, the best known of which are the rose- and 

 purple-variegated races of Delphinium and the miniature char- 

 acter of Drosophila virilis. The rose-variegated race is homozy- 

 gous for the recessive gene, rose-a.* Flowers of this race are 

 rose-colored but flecked with numerous dots and small spots of 

 purple. The rose color is the expression of the homozygous 

 rose-a gene, but each spot of purple is the result of a mutation of 

 one of the rose-a genes to its dominant allele for purple. When 

 a gene mutates to purple in a cell, that cell and all the others 

 which come from it by cell division will be purple, for apparently 

 the gene does not mutate back to rose-a. If a mutation occurs 

 just at the last cell division, the purple spot will include only one 

 cell. If it takes place at the division before the last, the spot 



* Mutant alleles were originally designated by the name of the mutant fol- 

 lowed by a Greek letter, as miniature-alpha, miniature-beta, and miniature- 

 gamma. Because of the inconvenience in typewriting manuscripts and the 

 added expense of printing that the use of the Greek alphabet entails, the 

 Greek letters have recently been replaced by Roman letters, as miniature-a, 

 miniature-b, and miniature-c. 



