Hybridization and Selection 



291 



Eggs 

 TS Tw dS 



dw 



A 

 TS 



Tw 

 Sperm 



dS 



dw 

 D 



Note that in square 6 there is represented a plant that has the 

 factors desired and no others. It is homozygous for both the 

 desired characters, tall plants and wrinkled seeds. There will 

 be twelve seeds out of every sixteen that will produce tall plants 

 in the third generation, and four that will produce dwarfs (ratio 

 3:1). Which one of the twelve will produce only tall plants with 

 wrinkled seeds can be determined only by planting the seeds of the 

 F2 generation and watching the characteristics and proportion 

 of tall and dwarf and smooth and wrinkled seeds developed 

 from each individual plant. If this is accurately done, the 

 plant breeder will be able to select the plants having the 

 composition represented by square 6 and his problem will be 

 solved. 



Referring now to the diagram at the right, all the plants repre- 

 sented by the squares along the axis AB are homozygous and will 

 breed true if self-pollinated. All the plants represented by the 

 squares on the axis CD are exact duplicates of the hybrid or Fi 

 generation and will produce all the sixteen possible combinations 

 when self-pollinated. These four squares represent plants that 

 are heterozygous for both pairs of characters. The remaining 

 eight squares represent plants that are homozygous for one 

 character but heterozygous for the other. By means of similar 

 diagrams it is possible to calculate the chances of securing a 



