290 General Botany 



The plant breeder starts by cross-pollinating the two plants, 

 a process which may be represented as follows : 



T S X d w 

 He wishes to secure a plant that will have the composition T w. 



Sperm Egg 

 T S X d w 

 All plants have TdSw in Fi generation 



When the plants of the Fi generation are mature, they will all be 

 tall and will have smooth seeds. 



He now allows these hybrids to self-pollinate and plants the 

 resulting seeds. Only one of each pair of contrasting characters 

 will be contained in each sperm and each egg from these plants. 

 It will be either the factor for tallness or for dwarfness, but not 

 both. It will be either the factor for smoothness or wrinkledness, 

 but not both. Hence, there will be four kinds of sperms and four 

 kinds of eggs, whose cornposition may be indicated as follows : 



Sperms Eggs 



TS TS 



Tw Tw 



dS dS 



dw dw 



Any one of these sperms may unite with any one of the eggs, and 

 if hundreds of zygotes are formed the several kinds of sperms 

 will unite with each of the several kinds of eggs in about equal 

 numbers, and there will be sixteen possible combinations, which 

 may be seen by writing out the following checkerboard diagram. 

 Write first the four kinds of eggs above, then the four kinds of 

 sperms on the left side. Then put in each square the combina- 

 tion of the factors carried by the egg (indicated above) and the 

 sperm (indicated to the left). At the right of this diagram we 

 may make a second diagram giving the characteristics of each 

 of the plants represented by the sixteen squares : 



