1924] Collins: Inheritance in Crepis capillaris (L.) Wallr. 277 



The very fact that they do not approach a common type under culti- 

 vated conditions supports the theory of the constancy of the gene and 

 is evidence of the inability of the habitat to induce genotypic changes. 

 The occurrence of duplicate genes in other plants has brought 

 forth the opinion that they ma.y indicate the presence of duplicated 

 chromosomes. Three cases of duplicate genes have been found in 

 Bursa (Shull, 1920), a plant having 32 chromosomes (4 X S), while 

 a case of triplicate genes is reported in a wheat (Nilsson-Bhle, 1909) 

 Avhich has 42 chromosomes. This number is three times the number 

 (14) found in several species of Triticum (Sax, 1921). Several pairs 

 of duplicate genes have been found in Crepis capiUaris. No plants 

 producing such ratios have been examined cytologically, but in no 

 visible way do they differ from plants which give 3 to 1 ratios for 

 the same characters. From what is known regarding the effect of 

 duplication of single chromosomes or of whole sets of chromosomes 

 in Datura (Blakeslee, 1922) and in Nicotiana (Clausen and Good- 

 speed, 1924), it is difficult to suppose duplication of chromosomes has 

 occurred here. That we have parallel mutations in identical loci 

 of two chromosomes of the same kind derived from a form with a 

 different number by some meiotic irregularity is equally improbable, 

 for capiUaris has but three pairs of chromosomes, no two similar 

 enough in size to be construed as duplicates. There are several 

 other ways to account for the appearance of duplicate genes, some 

 of which have been discussed by Shull (1918). Four of these possi- 

 bilities are {a) the occurrence of similar gene mutations in different 

 chromosome pairs; (&) the mating of non-homologous chromosomes; 

 (c) duplication of entire chromosomes; and {d) duplication of 

 sections of chromosomes. The possibility of a chromosomal dupli- 

 cation as the cause of the origin of duplicate genes in Crepis is very 

 unlikely, as has been shown above. The other possibilities cannot be 

 dealt with so readily. It would appear, however, that, had duplica- 

 tion of a section of a chromosome taken place, other characters, the 

 genes for which were located in the duplicated section, should show 

 similar inheritance ratios. As a matter of fact, two other characters 

 in Crepis capiUaris give ratios of 15 to 1, but in the one case tested 

 (revolute X smooth ribs) the type of linkage demanded by such an 

 hypothesis was not obtained. Mating of non-homologous chromosomes 

 should also result in duplication of other genes which should show 

 linkage relations. Although only a small amount of critical data is 

 as yet available, no confirmation of the linkage relations demanded 



