MUTATION IN MATTHIOLA 



299 



the peculiar phenomena depend on lethal factors or combinations of 

 factors originally due to mutation. 



Another possibility there mentioned, advanced by Belling (1914) 

 and since specially discussed by Goodspeed and Clausen (1917), is 

 that of the occurrence of lethal combinations of certain factors which 

 in other combinations may be in no way prejudicial to normal develop- 

 ment. As the latter paper shows, it is probable that in certain 

 crosses between ' ' good species ' ' most of the new combinations brought 

 together in the formation of P x gametes are incompatible with the 

 production of functional gametes. Perhaps in the case of Oenothera 

 there may exist within a species factors lethal in any combination 

 when homozygous, and other factors lethal only in certain com- 

 binations. 



A balanced-factor explanation for the inheritance of doubleness 27 

 in Matthiola, a case which Muller (1918) discusses, seems to have been 

 first definitely stated by Goldschmidt (1913), though he failed to pro- 

 vide for one feature of the evidence, the deviation of the heredity 

 ratio from 50 per cent. As has been shown (Frost, 1915), this 

 peculiarity may be due to greater viability of the homozygotes (sterile 

 doubles) during embryonic development, since the doubles are more 

 viable in the mature seeds and more vigorous in later development 

 (Saunders, 1915). In this case the "normal" factor is completely 

 eliminated in favor of the mutant (sterile-double) factor in the 

 formation of the sperms, and probably is partially eliminated in the 

 formation of either the eggs or the embryos or both. 



Here the normal singleness (sporophyll) factor D may act as a 

 lethal in the heterozygous parent, possibly from its general relations 

 of growth vigor in the presence of the more vigorous d-carrying cells. 

 If the lethal factor is situated in a distinct locus, evidently crossovers 

 are at most extremely rare. It is true that Miss Saunders (1911) 

 finds that Fj hybrids with pure single forms produce functional 

 single-carrying pollen ; with the pure single forms from which the 

 original "double-throwing" mutants arose, however, this might not 

 be true, or a lethal change may have occurred in the singleness factor 

 itself rather than in a factor coupled with it. The Drosophila case 

 would suggest a lethal change in another locus of the single-carrying 

 chromosome. 



In my paper of 1915 this lethal change in one chromosome ap- 

 parently accompanying the mutation of D to d in the homologous 



27 For a brief outline of the genetic behavior of doubleness see the discussion 

 of the experimental data for the smooth-leaved type. 



[157] 



