138 



Dihybrids, Trihyhrids, and Poly hybrids 



correct for a trihybrid as for a dihybrid, but is necessarily 

 more complicated and correspondingly less useful. In the hypo- 

 thetical case just mentioned, the Fi from any of the combina- 

 tions of different genotypes will be Aa Bb Cc. Each gamete will 

 contain three genes, one from each pair of alleles (plus, of course, 

 one of each of all the thousands more or less of pairs of genes 



P.: 



F,: 



AABBCC X aabbcc 

 AaBbCc 



F,: 



Cross: 



Aa Bb Cc X aa bb cc 



Fig. 47. The Fo and backcross of a trihybrid by the checkerboard 

 method. 



which make up the particular genotype but do not concern the 

 immediate problem). This fact is emphasized because students 

 frequently become confused in wTiting the gametes and place 

 the two genes of one pair of alleles in the same gamete, failing 

 to include a gene from one of the other pairs. The eight kinds 

 of gametes from the Fi are ABC, ABc, AbC, aBC, Abc, aBc, abCj 

 and abc, and they are found with equal frequency. In forming 

 a "checkerboard," these eight types, representing the male 

 gametes, would be placed at the top of the diagram, and the 

 same eight, representing the female gametes, would be in a col- 

 umn at the left. There would be sixty-four squares in the 



