66 Outline of Genetics 



presence of /; whereas the colored condition occurs only 

 when three conditions are simultaneously satisfied, 

 presence of R, presence of C, absence of /. 



Tetrahybrid problems, involving /, P, R, and C can be 

 worked out on the same principles as were previously outlined. 

 It is probably most convenient to consider the / factor first. 



UPpRrCc X iippRRCc. 



Considering the / factor, there will be | of the progeny with and 

 ^ without /. This first | is now dropped because it is bound to 

 be colorless on account of the presence of /, no matter what may 

 be the rest of the germinal composition; and in this case it is the 

 latter ^ that is eligible to be carried on for further consideration. 



The C factor gives f with and \ without C. Applied to the 

 \ that was carried on, this becomes ^Xf or f without / and with 

 C, and |Xi or | without / and without C. This last \ is hereby 

 dropped, since it lacks C and must be colorless; while the f is 

 carried on for further consideration. 



The R factor gives a ratio of i with:o without R. Conse- 

 quently no further fraction is dropped into the white phenotype 

 at this point, and the whole f is carried on. 



The P factor gives \ with and \ without P. Applied to the 

 I, this becomes f X| or A which lacks / and has C, R, and P, 

 and is therefore purple; and f Xf which lacks /, has C and R, and 

 lacks P, and is therefore red. 



Summarizing, | plus | or if is colorless, A is red, and A is 

 purple. The phenotypic ratio is, 3 purple 13 red: 10 white. 



liPpRrCc ''selfed." 



/ gives f with I:\ without /. Drop the f and carry on the \. 



C gives f with C:\ without C. Applied to the \ that was 

 carried on, this becomes |Xf or A which lacks / and has C; and 

 |Xi or j^,i which lacks / and C both. Drop the Vo and carry on 

 the A. 



R gives f with R : \ without R. Applied to the iV that was 

 carried on, this becomes i^Xf or if which lacks / and has both 

 C and R; and AXi or ^^ which lacks /, has C, and lacks R. 

 Drop the 6^4 and carry on the o^. 



