CALCULATING MENDELL\N EXPECTATIONS 107 



interesting facts. All the homozygotes expected lie in the 

 diagonal row of squares running from the upper left to the 

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

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

 only a single t;y'pe 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 + 

 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., wi^ only dominant characters 

 showing in the zygote, the fourHp|||||gwild appear as 9 AB : 

 3 Ab : 3 aB : 1 ab, the t^TDical^pi.^-b^fci^ratio. Let the 

 reader make out the checkerb^^^ ^i^^^^Bly these >state- 

 ments. % ^f^ 



In a similar way c^fie ma^ calculate^i^H||k' algebra |>r by 

 checkerboard the F2 expected result fjoM^^rihybrid cross. 

 The eight kinds of gametes which theJp|H^|eterozygous Fi 

 individuals would produce have already ^een indicated, viz., 

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



By the checkerboard me^pd, each combination would 

 be found homozygous (unite^^ith a gamete like itself) in 

 a different square of the diagonM of th^nfigure, 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 expdcted to produce. The 

 subsequent calculation is easy and certain. One soon learns 



