316 CENOTHERA AND THE MUTATION THEORY 



An application of the principle of balanced lethals 

 to O. Lamarckiana indicates that the chromosome 

 pair that contains the balanced zygotic lethals (i.e., 

 that is responsible for the differences between Iseta 

 and velutina) is the one that is most important in 

 the production of the '^mutant" types. It is 

 apparently in this pair of chromosomes that nanella, 

 oblonga, rubrinervis, blandina, simplex, erythrina, 

 and others differ from typical Lamarckiana. If we 

 regard Lamarckiana itself as heterozygous and write 



its formula ■ — t—tt — ■ it is possible to write similar 

 veiutma 



formulae for the other types named, as has been 

 done in a slightly different fashion by deVries 

 himself. The ''Iseta" and "velutina" are not to be 

 thought of as single allelomorphs, but rather as 

 whole homologous chromosomes that differ in a 

 number of genes. In the formulae below the other 

 terms may be thought of as follows : 



oblonga — mostly Iseta, but modified either by crossing over 



or by mutation, 

 deserens — like Iseta in most respects, 

 decipiens — also like Iseta. Differs from deserens only in not 



having a gene for brittleness. 

 blandina — smooth leaved; otherwise like velutina. 



None of these, except oblonga, carries a lethal; the 



lethals have presumably been lost by crossing over. 



The terms just given apply to the chromosomes; 



the plants themselves may be represented as follows : 



