214 BOTANY: PRINCIPLES AND PROBLEMS 



which characters are transmitted to the second and later genera- 

 tions following a cross. The hybrid offspring arising from a 

 cross between a plant of a purple-flowered race and one of a white- 

 flowered race are, as we have said, all colored. In appearance 

 they resemble rather closely the purple-flowered parents, but in 

 most such crosses the hybrids are somewhat paler than the pure 

 colored types. When two of these hybrid colored plants are 

 crossed, or when one of them is self-fertilized (which amounts to 

 the same thing genetically) both colored-flowered and white- 

 flowered plants appear in their offspring, the former constituting 

 about three-fourths and the latter about one-fourth of the total 

 number of individuals in the progeny. These white-flowered 

 plants breed perfectly true when self fertilized, and purple flower 

 color never appears in subsequent generations of their descendants 

 when inbred. A part of the colored plants (approximately one- 

 third of them) breed perfectly true to the purple color, none of 

 their offspring, when inbred, possessing white flowers. The rest 

 of the colored-flowered plants, however, (about two-thirds of 

 them and thus about one-half of the total number of the offspring) 

 resemble the hybrids in color and behave when self fertilized 

 exactly as the hybrids did, producing offspring of which three- 

 fourths are colored and one-fourth white. These facts are set 

 forth diagrammatically in Fig. 123. * This separation and sorting 

 out of characters which occurs in offspring of hybrid plants is 

 known as segregation. The discovery and interpretation of 

 segregation were perhaps the most important contributions which 

 Mendel made to our knowledge of inheritance. 



The essential character of segregation is shown in the beha- 

 viour of contrasting factors when they exist together in a hybrid 

 individual. A factor transmitted through the gametes of one 

 parent and a contrasting factor transmitted through the 

 gametes of the other parent, come together and coexist in the 

 cells of the hybrid offspring plant without blending or losing their 

 identity; and when such a hybrid plant produces its own sexual 

 cells, in turn, the two factors become completely separated or 

 segregated from one another, each of the new gametes containing 

 either the one or the other but never both. This is well illustrated 

 by the example which we have been using. The factors for 

 purple and for white flower color must both be present in the 



* The first generation following a cross is technically known as the 

 Fi (first filial generation), the second as the F2, the third as the Fj, and so on. 



