CHROMOSOMES AND MEN DELI AN HEREDITY 



289 



are now possible: a T-sperm with a T-egg, a T-sperm with a ^-egg, a 

 /-sperm with a T-egg, and a /-sperm with a /-egg. These four combina- 

 tions result respectively in a tall plant (pure dominant, TT), two tall 

 hybrids {Tt), and a dwarf plant (pure recessive, //). It is obvious that 

 in the long run these three types will tend to occur in the ratio of 1:2: 1. 

 The pure tall individuals and the tall hybrids in F^ are ordinarily 

 distinguished from each other by the "back-cross test. " It will be readily 

 seen that when a pure tall plant {TT) is crossed with the pure recessive 

 type (//), all of the offspring will be tall {Tt); whereas, when a tall hybrid 

 {Tt)\s crossed with //, half of the offspring will be tall {Tt) and half will be 

 dwarf (//). 



MlRAfilLU JALAPA 



Fig. 166.— Inheritance of flower color in Mirabilis. See text. {Adapted from. Correns.) 



The Mendelian proportion of hybrids and pure types is perhaps better 

 illustrated by characters in which dominance is imperfect or lacking. 

 In Mirabilis jalapa, for example, the hybrids are more or less intermediate 

 with respect to flower color and are easily distinguishable from the pure 

 parental types (Fig. 166). When plants bearing pure crimson flowers are 

 crossed with those bearing pure white flowers, the hybrid plants of the 

 Fi generation have magenta flowers. When these hybrids are bred 

 among themselves, the resulting F2 generation comprises plants of three 

 visibly different types: pure dominants with crimson flowers, hybrids 

 with magenta flowers, and pure recessives with white flowers; and these 

 types tend to occur in the ratio of 1 :2:1.^ 



Mendel's researches on peas included also a study of six other pairs 

 of heritable characters (now known as allelomorphic pairs), the two 



^See further Kiernan and White (1926) on this case. 



