CALCULATING MENDELIAN EXPECTATIONS 107 



interesting facts. All the homozygotes expected lie in the 

 diagonal row of squares running from the upper left to the 

 lower right comer of the figure. Compare Fig. 49. These 

 are the individuals that will "breed true," i. e., will fonn 

 only a single type of gamete. They are four in number, each 

 of a different sort and would result from the union of two like 

 gametes of each of the four expected types, AB + Ab + aB -f 

 ab (or in Fig. 49, EA + Ea + eA + ea). They represent all 

 the possibilities as regards true breeding ('*fixed") forms to 

 be expected from the cross. What the nature of the other 

 individuals to be expected would be would depend upon 

 the completeness of dominance. If dominance should be 

 complete, heterozygotes would be indistinguishable except 

 by breeding test from the four expected homozygotes; other- 

 wise homozygotes and heterozygotes might be distinguish- 

 able by appearance as well as by breeding tests. With 

 complete dominance, i. e., with only dominant characters 

 showing in the zygote, the four sorts would appear as 9 AB : 

 3 Ab : 3 aB : 1 ab, the typical dihybrid F2 ratio. Let the 

 reader make out the checkerboard and verify these state- 

 ments. 



In a similar way one may calculate, either by algebra or by 

 checkerboard the F2 expected result from a trihybrid cross. 

 The eight kinds of gametes which the triply heterozygous Fi 

 individuals would produce have already been indicated, viz., 

 ABC -F ABc + AbC + aBC + Abe -f aBc + abC + abc. 



By the checkerboard method, each combination would 

 be found homozygous (united with a gamete like itself) in 

 a different square of the diagonal of the figure, and hetero- 

 zygotes containing the same dominant characters would be 

 found elsewhere in the table sufficient in number to bring the 

 totals up to 27 ABC : 9 ABc : 9 AbC : 9 aBC : 3 Abc : 3 aBc: 

 3 abC : 1 abc. This is the typical trihybrid F2 ratio, when 

 complete dominance exists. 



To repeat, it is all essential to determine first the kinds of 

 gametes each parent to a mating is expected to produce. The 

 subsequent calculation is easy and certain. One soon learns 



