INDEPENDENT MEN DELI AN INHERITANCE 



99 



down at one side and at the top of the checkerboard and any square is 

 filled out by writing down the genetic formula of the gamete at the top 

 of its column and the one at the end of its row. If the series are written 

 in the order shown the diagonal 1-3 will pass through all homozygous 

 combinations. The number of these is evidently equal to the number 

 of possible combinations in the gametic series. The diagonal 2-4 

 passes through all those combinations in which all three pairs of factors 



1 ABC .ABc AbC Abo aBC aBc abC abo 2 



ABC 



f ABc 



AbC 



Abe 



aBC 



aBc 



abC 



ale 



3 



FIG. 48. Checkerboard method of analyzing expected results in Fz from a cross involving 

 three pairs of allelomorphs. The "a;" zygotes belong to phenotype ABC. Cf. p. 98. 



are heterozygous, and the number of these is also equal to the number of 

 kinds of gametes. The bottom row gives the ratio which would be ob- 

 tained by crossing the FI back to the triple recessive form. The student 

 will be able to determine other relations existing in checkerboards of this 

 type. 



From a mathematical standpoint, students of genetics are interested 

 in two things, the number and proportion of various types of individuals, 



