238 The Nature of Gene Mutations 



will be formed from two cells. In other words, the earlier the 

 mutation occurs in the development of a flower, the larger the 

 purple patch. Therefore, the time at which the mutation oc- 

 curred can be determined fairly accurately from the size of the 

 mutated area. 



All these mutations are somatic mutations in cells of the 

 flower, but Demerec has found evidence that this same gene mu- 

 tates in germ cells as well. When a rose-a plant was self- 

 fertilized, most of the offspring had typical rose-a variegated 

 flowers. Some, however, had solid purple flowers, and a small 

 percentage had large purple sectors or chimeras in an otherwise 

 normal rose-variegated flower. The purple-flowered plants are 

 believed to have arisen as the result of gene mutations in the 

 formation of germ cells. If one rose-a gene mutated to purple 

 at that stage, one germ cell would have the dominant gene for 

 purple and a plant which arose from it would have purple 

 flowers. Plants with large purple chimeras in the flowers are 

 believed to have resulted from somatic mutations very early in 

 the development of the flower. In the sepals of normal rose- 

 variegated plants, the rate of mutability of the rose-a gene was 

 practically the same during the last twelve cell generations in 

 the development of the flower. In gametogenesis, the rate was 

 found to be 267 per million cells, a value close to that for the 

 rate of mutability in the sepals. It is interesting to note that 

 this rose-a strain originated from a plant which was heterozy- 

 gous for rose-a and lilac. Since all the subsequent plants came 

 from this one, all w^ere the descendants of one rose-a gene. 



Another gene that is unstable in Delphinium ajacis is the 

 lavender-a (or lavender-alpha) gene. Lavender-variegated 

 flowers have numerous small spots of purple on a lavender back- 

 ground. These spots are the result of many somatic mutations 

 of the lavender-a gene to the dominant allele for purple in plants 

 homozygous for that recessive gene. That all the dots are small 

 and of approximately the same size indicates that this gene has 

 a high rate of mutability towards the end of the development of 

 the sepals and petals. Differing from rose-a, lavenderra flowers 

 have no large or intermediate-sized spots or streaks. This 

 absence of any purple spots other than small dots indicates that 

 the gene is stable or that it mutates at only a very low rate 

 during all but the last stages of development of the flower. Like 



