62 Outline of Genetics 



These principles reveal the nature of supplementary 

 factors. Unlike complementary factors, they never 

 produce characters, but merely modify characters already 

 present. Otherwise they follow the same principles of 

 independent Mendelian inheritance with which we are 

 now familiar. 



The student will find a very useful exercise in the solution of 

 various problems which involve simultaneously several types of 

 factors. In solving such problems, it is neither necessary nor 

 advisable to employ any "rule-of-thumb" method. As usual 

 the checkerboard can be depended on to provide an accurate solu- 

 tion, but this is too slow and cumbersome. Instead, the problems 

 can always be solved rather easily by keeping in mind, at all times, 

 the fundamental mechanism of inheritance that is at play and 

 proceeding with the solution in a logical, orderly manner. (If 

 the student simply remembers the Mendelian mechanism he can 

 really work out his own methods of solving the problems.) 



In attacking polyhybrid problems, the first principle to 

 remember is to work out separately the solution for each pair of 

 allelomorphs, and then to put together these monohybrid solu- 

 tions. This ''putting together," in the case of simple determiners, 

 amounts merely to multiplication, as was described before, but 

 where factors are at play other methods of putting together are 

 necessary. In every case, the method of putting together the 

 monohybrid solutions is a rather obvious one, and is clearly indi- 

 cated by the definition of the factor type with which one is dealing. 



A few examples involving the P, R, and C factors will serve to 

 illustrate. In these cases, one must consider the P factor last, 

 since it is effective only when both R and C are present. 



PPRyccXPpRRcc. 



Considering first the C factor, one finds that a o: i ratio will result; 

 that is, all of the progeny will lack the C factor. Since this is the 

 case, one need go no farther with the solution, since absence of C 

 is sufiicient in itself to insure that all of the progeny will be color- 

 less, or white. 



PpRrCcXppRrCC. 



