INHERITANCE— IMPROVEMENT BY BREEDING 183 



numbers. The heterozygous plant therefore gives rise to chap. 

 ovules comprising equal numbers of gametes of the four dif- 

 ferent types AB, Ab, aB, and ab, and also to pollen grains 

 comprising the same four types. When the heterozygous 

 plant is self-pollinated, an ovule of any one of these four types 

 has an equal chance of fertilization by a pollen grain con- 

 taining any one of these four types. The result is mechanic- 

 ally demonstrable by the " chess-board " method of plotting, 

 as shown in Table XXXIII. Each of the four terms AB, Ab, 

 aB, and ab of the gametic series, is first written four times 

 horizontally across the table, from left to right ; it is then 

 written four times vertically, from top to bottom, care being 

 taken to follow the same order. In this simple mechanical 

 way all the possible combinations are represented, and in their 

 proper proportions. 



The following are the possible combinations in which the 

 gametes would segregate in a dihybrid : — 



Table XXXIII. 

 DISTRIBUTION OF GAMETES IN A DIHYBRID. 



Thus there are nine cases in which both dominants (A and 

 B) meet ; three contain the dominant A, without B ; three 

 contain B without A, and one carries the two recessives a and 

 b, without either dominant. This gives the ratio : — 



9:3:3:1: 



or 56'25 : 18-75 : T S'75 : 6'25 per cent. 



