304 Cell as Basis of Organic Activity 



In the cross illustrated in Fig. 93, the following ratios are observed. 



1 



2 

 2 

 1 

 4 

 2 

 1 

 2 

 1 



9 

 3 

 3 

 1 



This 9 :3 :3 :1 phenotypic ratio is characteristic of the F2 generation 

 of a dihybrid cross where dominance is exhibited in both sets of genes 

 and where they are located on different chromosomes. 



Multihybrid Crosses. — Crosses involving more characters can be 

 made, but they become very awkward to handle. A cross similar to the 

 ones just studied but involving three characters would have the Fa 

 phenotypic ratio of 27:9:9:9:3:3:3:1. This would involve 64 squares 

 in the Punnett diagram. The large number makes it very awkward to 

 study diagrammatically these multihybrid crosses ; however, a relation- 

 ship exists so that formulas may be constructed to indicate the number 

 of gametes, combinations, phenotypes, etc. 



Let N = the number of pairs of factors to be studied, then 2" 

 = the number of gametes produced by each Fi. 2° also equals the 

 number of phenotypes of the F2 generation; 3° equals the number of 

 genotypes; and 4° equals the number of squares in the F2 Punnett dia- 

 gram. 



Formula 2 pairs 3 pairs 4 pairs 



Number of gametes produced by F^ 2" 4 8 16 



Number of different phenotypes in Fj 2° 4 8 16 



Number of different genotypes in Fj 3" 9 27 81 



Number of squares in F, Punnett square 4° 16 64 256 



Interaction of Factors. — There are certain individual phenotypic 

 characters which are controlled by more than one pair of genes. By 



