INHERITANCE 



295 



Tall 



Yellow 



Round 



Dwarf 



Green 



Wrinkled 



3 



% 



SYR 



sYR 



SyR 



syR 



SYr 



sYr 



Syr 



syr 



Fig. 184. — Diagram of a Mendelian trihybrid. Results of crossing tall 

 Peas bearing yellow round seeds (SYR) with dwarf Peas bearing green wrinkled 

 seeds (syr). The circles represent the zygotes and the characters of the soma 

 (phenotype); the letters within the circles, the germinal constitution (geno- 

 type). The letter groups outside the recombination square represent the gam- 

 etes. The Fi hybrids form eight types of gametes, giving sixty-four pos- 

 sible types of zygotes, representing eight phenotypes (shown graphically) and 

 twenty-seven genotypes. There is one pure dominant (in upper left corner) 

 and one recessive (in lower right corner). Eight are homozygotes (diagonal 

 from upper left to lower right corner) and the rest are heterozygotes. The 

 zygotes in the diagonal from upper right to lower left are identical with the 

 Fi generation. The relative number of individuals composing the phenotypes 

 is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. 



