448 



THE MAINTENANCE OF SPECIES 



BOARD 



combinations, falling into four phenotypic groups, and nine genotypic 

 groups, as shown in the following checkerboard, in which the double 

 gametes of the dihybrid parents are represented outside the double 

 hnes, and their combinations in the offspring indicated within the 

 sixteen squares. The arbitrary symbols used are, B (black) ; b (chest- 

 nut) ; T (trotter) ; t (pacer). It will be seen from the checkerboard that 

 DIHYBRID CHECKER- ^^^^ chances out of sixteen are possible that 



a black trotter will result, since both B and T 

 are present at least once in their make-up. 

 There are three chances that a black pacer 

 (Bt) will occur, three chances for a chestnut 

 trotter (bT), and one chance in sixteen that 

 the dihybrid parents will produce a chestnut 

 pacer (bt) . Thus, the phenotypic ratio in the 

 case of a dihybrid is typically 9:3:3:1. The 

 checkerboard further shows that the sixteen 

 possibilities fall into nine genotypic, or ac- 

 tually different, groups represented by dif- 

 ferent combinations of the four symbolic letters within the squares. 

 The expectation when two trihybrids are crossed is shown by 



TRIHYBRID CHECKERBOARD 



