226 



The Nature of and Changes in Genes 



mutations or "sports" (Fig. 71) and suggested that these new 

 types are a greater factor in evolution than the small, gradual 

 changes which Darwin had thought were so important. Since 

 that time, mutations have been discovered in a great many 

 plants and animals. 



Fig. 71. Buds and flowers of the evening primrose, (a) Flower of 

 Oenothera Lamarckiana. (b) Mutant substella of Shull; note the style and 

 stigma above the small petals, (c) Mutant confusa of Shull; in this mutant 

 the stigmas and stamens are massed together in such a way as to occupy 

 completely the lumen of the bud cone at the stigmatic level forming a 

 barrier to the elongation of the petals; the buds open broadly between two 

 sepals, merely split between the others, and the petals never spread. These 

 two mutants are new. (Courtesy of Dr. G. H. Shull.) 



Many of the inherited new types that have appeared in 

 various organisms were subsequently shown to be recombina- 

 tions of previously known genes, such as the "outside-in" type 

 of flower in the Evening Primrose which resulted from the com- 

 bination of the recessive genes supplena (double flowers) (Fig. 

 72) and brevistylis (short style) , never previously together in the 

 same plant (Fig. 73). Other new inherited types have resulted 

 from changes in the number of chromosomes or from rearrange- 

 ments of, losses of, or duplications of chromosomal segments, and 

 they have been classified as anomozygous mutations. Still other 



