Hybridization and Selection 289 



Mendel concluded from these and other experiments that most 

 characters that make up a plant are each inherited independently 

 of others. He also concluded that each egg and sperm carried 

 but one of two contrasting characters, while the zygote which 

 is formed by the union of the sperm and egg will, therefore, be 

 either homozygous (containing two similar factors) or hetero- 

 zygous (containing two contrasting factors). In the latter case 

 the plant that develops will show the dominant character but 

 not the recessive. Its offspring, however, will show both char- 

 acters, three fourths being like the dominant and one fourth 

 being pure recessives. 



The combinations of characters that will appear in the progeny 

 in the F2 generation above may be shown more clearly by a dia- 

 gram. 



Application of Mendel's laws. Just how Mendel's laws may 

 be applied to a particular breeding problem will be apparent if we 

 follow the steps in the making of a hybrid with two pairs of con- 

 trasting characters. Suppose we have a tall pea with smooth 

 seeds and a dwarf pea with wrinkled seeds. Tall peas require a 

 longer time to develop and produce more peas per plant. Dwarf 

 peas are, therefore, earlier. Wrinkled peas are generally sweeter 

 than smooth peas because they contain more sugar. For these 

 reasons it is decided that a tall pea with wrinkled seeds would 

 be a desirable variety to produce. How can the plant breeder 

 secure this combination from the two plants in his possession ? 

 Knowing that tallness (T) is dominant over dwarfness (d) and 

 that smoothness (S) is dominant over wrinkledness (w), the 

 problem is a rather simple one. 



