No. 586] THE INHERITANCE OF DOUBLENESS 627 



determined by one dominant factor, S ; the eggs of the 

 double-thrower, then, are SF ("single") and sF 

 ("double"). He assumes, also, that half the pollen- 

 grains or microspores, in all races of Matthiola, lack F, 

 probably because of elimination of part of an X-chromo- 

 some, and that these -pollen-grains degenerate or at least 

 are non-functional, so that no staminate plants are pro- 

 duced. It is necessary to assume, then, that in the 

 double-throwing races S (or s) and F are carried by the 

 same chromosome, and that the S-carrying chromosome 

 is always the one to eliminate F. The S-carrying chro- 

 mosomes will then be the ones destined to degenerate. 

 The pollen resulting is of two kinds, Sf (" single," non- 

 functional), and sF ("double," functional). 



The double-throwing plant, then, is SFsF; its eggs are 

 SF and sF, while its pollen-grains are Sf (non-func- 

 tional) and sF. Self-pollination gives, then, SFsF 

 (heterozygous or double-throwing singles) and sFsF 

 (homozygous sterile doubles). 



The factor S, however, can not in itself, in general, in- 

 sure pollen-degeneration, since homozygous singles (SS) 

 produce fertile pollen. Nor can the case be one of degen- 

 eration of all pollen-grains receiving a maternal X-eJim- 

 mosome, as is proved by the results of crossing SS and Ss 

 races. Heterozygous singles (Ss) which get the S factor 

 from a pure single (SS) parent, either through egg or 

 through sperm, produce good S pollen, as is shown by the 

 ordinary Mendelian ratio among their progeny (1 homo- 

 zygous single (SS) :2 heterozygous singles (Ss) :1 homo- 

 zygous double (ss). Goldschmidt is driven to assume, 

 therefore, that the singleness factor (Sj) in the double- 

 throwers differs from that in the pure singles (S) — or 

 else to suppose that another factor interferes in the 

 former type. 



It will be seen that Groldschmidt gives, at most, only an 

 indefinite implied explanation of the deviation of the 

 double-single ratio from equality in the double-throwing 

 races. And it is hard to see what advantage is secured 



