158 University of California PuMications in Agricultural Sciences \ Vol. 4 



chromosome was considered puzzling. Evidently, however, it may 

 have occurred in one chromosome before D mutated to d in the other, 

 and even then may have produced its lethal effect. It is evident 

 that if doubleness should arise in the absence of the lethal effect it 

 would tend to be eliminated by the return of one-third of the singles 

 to the homozygous condition in each generation. In fact, it is possible 

 that the lethal change arose later than doubleness, as in the Droso- 

 phila case, or was brought in later by cross pollination, and happened 

 to be preserved as a result of horticultural selection for a high pro- 

 portion of doubles. 



A parallel-column comparison between the double type and the 

 types especially discussed above has already been given, in connec- 

 tion with the smooth-leaved type. It will now be seen that this com- 

 parison seems to apply to all mutant types, except early, that have 

 been genetically tested, the principal differences between these types 

 relating to the heredity percentage and the apparent presence or 

 absence of linkage with the single-double factors. 



From the standpoint of its relation to genetic analysis the double- 

 ness factor is remarkably similar to the sex factor in animals. There 

 are two types in each generation, one heterozygous and the other 

 evidently homozygous, and these types are produced by the fertiliza- 

 tion of two kinds of eggs, produced in equal or nearly equal numbers, 

 by a single kind of sperm. Although one of the somatic types is 

 sterile, and the uniformity of the sperms produced by the other is due 

 (evidently) to lethal action, the opportunity for chromosome analysis 

 is similar to that with sex chromosomes. 



We may say that the doubleness factor and its normal allelomorph 

 {d and D) are carried by chromosome pair I. Already we know 

 several other pairs of factors evidently carried by this pair of chromo- 

 somes. These are, to name only the mutant or possibly mutant 

 member of each pair of factors: P (pale sap color) and W (colorless 

 plastids), both studied by Miss Saunders (1911. lOlla) ; C (erenate- 

 leaved), S (slender; possibly two factors), and N (narrow-leaved). 

 As we have seen, the last three of the-se are probably lethal when 

 homozygous, and one or more unidentified lethal factors may be con- 

 cerned in the breeding results, while the doubleness factor affects the 

 race much like a recessive lethal, since all dd individuals are completely 

 sterile. 



