278 University of California Publications in Agricultural Sciences [Vol. 2 



by these two methods of gene duplication has been obtained. Shull 

 rejected the idea of the occurrence of two independent mutations as 

 a cause of duplication of genes in Hursa on the ground that the char- 

 acters were of such a complex nature tliat the occurrence of two 

 independent mutations producing identically the same somatic results 

 was on the verge of impossibility. The characters in Crepis for which 

 there are duplicate genes cannot be considered as complex, and the 

 occurrence of similar mutations in non-homologous chromosomes 

 therefore seems at the present time to be the more reasonable explana- 

 tion of the origin of duplicate genes in this species. 



Sturtevant (1921) has shown that some points in the germinal 

 material of a given species are more susceptible to mutations than 

 others. There is evidence that such a mutating locus occurs in 

 capiUaris, for the same character, bald, has appeared in a number of 

 strains derived from widely separated localities. The identity of 

 these genes for bald has been proved in all cases except one (France) 

 by crosses in which they proved to be allelomorphic. That a certain 

 locus may mutate in the same w-ay in other species is at least indicated 

 by the fact that this character is now known to occur in four other 

 species, none of which has been grown extensively among our cultures. 

 The gene for bald is recessive in capillaris and is also recessive in the 

 species cross, setosa X capillaris. 



No less interesting and unique is the group of complementary 

 genes found in C capillaris where the appearance of three such pairs 

 of genes are concerned with the inheritance of leaf characters and a 

 fourth with chlorophyll. It is not strange, however, that a greater 

 number of complex gene relations should be encountered in a species 

 containing a low number of chromosome pairs than in species having 

 a larger number, unless the larger number results from reduplication. 

 There is probably a minimum number of genes which is necessary 

 in any species, and there is no reason to believe, a priori, that a species 

 with a larger number of chromosomes need have a correspondingly 

 larger number of genes. There is also evidence from Drosophila that 

 the genes are distributed at random in each chromosome (except in 

 cases of multiple allelomorphs) and among the chromosomes. When 

 tills basic number of genes is distributed among a large number of 

 chromosomes, more characters will show simple types of inheritance. 

 When this basic number is distributed in a fewer number of chromo- 

 somes, there will necessarily result more complex types of inheritance. 



