mutant obtained from it had white eyes (Morgan, '10), 

 and this character proved to be a simple sex-linked reces- 

 sive. From the white-eyed form arose a fly with eosin 

 eyes ( Morgan, '12). This new character was found to be 

 a sex-linked dominant to white, and a sex-linked recessive 

 to red. Finally, there arose a form with cherry eye color 

 (Safir, '13). This has the same relation to red and to 

 white as has eosin. Mated to eosin it gives an inter- 

 mediate color, which splits up into cherry, intermediate, 

 and eosin in F 2 . The nomenclature adopted in this case 

 is as follows : 



Allelomorph present in the red-eyed fly, If*. 

 Allelomorph present in the white-eyed fly, u: 



Allelomorph p P -em in the .-hen y-eyed fly, tt' c . 



Trow ('13) has suggested the possibility of an asym- 

 metrical reduplication series, giving a gametic series of 

 wAB:xAb:yaB:zab, where w need not equal z, nor 

 x equal y. It should be noted that an actual demonstra- 

 tion of such a ratio, or of its non-existence, is almost ex- 

 cluded for the reason that it would be practically impos- 

 sible to be sure one was not dealing with a case involving 

 differential viability. However, perhaps the most stri- 

 king general fact brought out by the study of linkage is 

 that each pair of linked genes (allelomorphs), considered 

 separately, follows a perfectly regular Mendelian course. 

 I think we are, therefore, justified in assuming that the 

 number of gametes bearing A is always equal to the num- 

 ber bearing a, and similarly for B and b. Then, in Trow's 

 asymmetrical series, 



iv + x = y + z, 



w = z and x = y. 



In all that follows I shall assume that the reduplication 

 series are always symmetrical. On this assumption it 

 becomes unnecessary to consider the two halves of the 



