68 Inheritance and Evolution in Oii/iojttcra IV 



possible combinations with the two gametes of the male give eight 

 classes of young. However, (9.2), DEV\ x i)£^, produced six classes of 

 young, which meets the expectation, because when all the possible 

 combinations have been made the classes DE and DEH will each have 

 occurred twice. Homozygotcs, or heterozygotes containing H, are better 

 analyzed by mating with lioniozygous individuals as has been done in 

 (9.15) and (9.27), FF x DFH, and a number of others. 



Thy mating (9.18), DFH x DFH, is of intei-est bocau.se among the nine 

 possible combinations of gametes we should obtain three combinations 

 in which H occurs twice, viz. DHDH, DHFV\, and FV\FV\ ; i.e. these 

 individuals should have a double dose of H. In the sense that when 

 inbred, all of the gametes will receive the factor (or factors) for H, they 

 will be homozygous for it. It is interesting to note in this connection 

 that a number of individuals appear in such matings as this which have 

 the reddish pigmentation in an appreciably more intense condition than 

 other individuals containing H in the same culture or other cultures. 

 That such individuals may have a double dose of H is shown by matings 

 (38), CG X DHEW, and (43), GC x EHEH ; (38) producing CDH 23 and 

 GEH 20 ; and (43) producing GEH 90. 



Homozygotic individuals with a single dose of H, as regards the 

 characters in question, always give two sorts of gametes, while hetero- 

 zygotic individuals with a single dose of H always give four sorts of 

 gametes. Homozygotic individuals with a double dose of H always 

 give one sort of gametes, while heterozygotic individuals with a double 

 dose of H always give two sorts of gametes. 



It is seen that while the zygotic constitution of e.g., DFH is either 

 D-.FHovF: Z>H, its gametic formula is always D:F:DH: FH. 



All the possible combinations of such a mating as has just been 

 described may be obtained in the usual manner from the " 16-square" 

 as for an organism differing in two " indej)endent " characters, oi' they 

 may be derived in the f illnwing manner and the ratios be made some- 

 what more apparent : 



D : F: UH:FH x D: F: IM : FH 



