52 Outline of Genetics 



The checkerboard is an excellent method of depicting the 

 mechanism at play, but the same results can be obtained much 

 more quickly and just as safely by another method. Dihybrid 

 (and other polyhybrid) ratios can be obtained by multiplying 

 together the ratios of the monohybrid components (for this really 

 amounts to the same thing as the checkerboard). In the present 

 case, one monohybrid component gives an F2 ratio of 3 tall:i 

 dwarf, while the other monohybrid component gives 3 red : i 

 white. (3 tall: I dwarf) times (3 red:i white) equals 9 tall, red 13 

 dwarf, red 13 tall, whiten dwarf, white. 



The student will find it a very helpful exercise to work out 

 for himself the various phenotypic ratios that will be produced by 

 inbreeding the various genotypes shown in the Fa checkerboard, 

 and by crossing them in various ways. In such problems the 

 method of procedure is to work out separately the results for the 

 different monohybrid components and then to synthesize these 

 results. The synthesis in this case amounts to simple multiplication. 



First of all, one must familiarize himself with the various 

 possible ratios encountered in connection with monohybrid crosses. 

 These are indicated in the following simple table. (In every case 

 the student must satisfy himself as to how the Mendelian mechan- 

 ism brings about these results.) 



Phenotypic ratio 

 ^^°^^ Red White 



i?i?Xanything i : o 



Rr XRr 3:1 



Rr Xrr i : i 



rr Xrr o : i 



The solution of dihybrid problems then becomes a simple 

 matter. For example, take the following cases: 



TtRrXttRR. 

 The monohybrid ratio as regards height is i tall : i dwarf, while 

 the flower color ratio is i red:o white. Multiplying these together 

 gives the dihybrid result, i tall, red: i dwarf, red. 



TtRrXTtrr. 

 The height ratio is 3 tall:i dwarf, while the color ratio is i red:i 

 white, which results in the dihybrid ratio, 3 tall, red:i dwarf, 

 red: 3 tall, white: i dwarf, white. 



