170 



FUNDAMENTALS OF CYTOLOGY 



122 by T) which tends to make the resulting plant tall. The germ cells 

 of the dwarf carry something {t) causing the dwarf condition. In the 

 first hybrid generation {h\) both factors are present, T coming from one 

 parent and t from the other, but T dominates and prevents the expression 

 of the recessive i so that the plants of this generation are all tall. When 

 the hybrid (Fi) produces germ cells, the two factors for tallness and 

 dwarfness segregate, half the gametes receiving T and the other half 

 t. Each gamete therefore carries either one or the other of the two 

 factors in question but never both; it is pure either for T or for t. This 

 segregation in the germ cells of factors associated throughout the soma 

 is the central feature of the entire series of Mendelian phenomena and 

 is the basis of Mendel's first law. Since the gametes, both male and 

 female, produced by the hybrid plants of the Fi generation are of two 

 kinds (half of them bearing T and half bearing t), four combinations are 



Parents 



I 



PureTall 

 TT 



Pure Dwarf • 



F. 



Toll Hybrid 

 Ti 



Tall Hybrid 

 Tt 



Gameies Fj 



I 1 



PureTall TT - Breeds true 

 Tall Hybrid Tt - 3> ■ \ 



Tall Hybrid Tt - 3 \ 



Pure Dwarf tt - Breeds true 



Fig. 122. — A typical case of Mendelian liei'edity in the garden pea. 



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 combinations result, 

 respectively, in a tall plant (pure dominant, T T), two tall hybrids (T t), 

 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 F2 are ordinarily 

 distinguished from each other by the testcross, or backcross test. It will 

 be readily seen that when a pure tall plant {T T) is crossed with the pure 

 recessive type (/ /) all the offspring will be tall (T t); whereas, when a tall 

 hybrid (7" /) is crossed with / /, half the offspring will be tall {T /) and 

 half will be dwarf (/ /). 



The ]\Iendelian proportion of hybrids and pure types is perhaps better 

 illustrated by characters in which dominance is imperfect or lacking. 

 In four-o'clocks, for example, certain hybrids are more or less inter- 

 mediate with respect to flower color and are easily distinguishable from 

 the pure parental types. 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 



