392 THE NEW INDIVIDUAL Part IV 



Mendel went on rearing the plants to see if their inherited content, the 

 genotype, was what it appeared to be, that is, the phenotype. By analyzing the 

 offspring of self-pollinated plants of the F2 generation he found that one- 

 fourth of them were pure tails, one-fourth pure dwarfs, and one-half ap- 

 parently tall but actually hybrids. When crossed with one another these hybrids 

 produced a 3:1 ratio of tall dominants and dwarf recessives as before. 



Mendel's Explanation 



He explained his observations by assuming that all living things transmit 

 hereditary traits by means of physical particles in the sex cells of the parents. 



Parents Tall (tall) X Dwarf (dwarf) 



F, Tall (dwarf) X Tall (dwarf) 



1 



F2 Tall (tall) Tall (dwarf) ^ Tall (dwarf) Dwarf (dwarf) 



Fig. 20.3. The results of Mendel's cross of garden peas of pure ancestry for 

 tallness with peas of pure ancestry for dwarfness. The first generation, first filial 

 Fj, was tall; the second generation, Fo, was tall in a proportion of three tails to one 

 dwarf. Mendel named these characteristics dominant and recessive, terms used 

 ever since. In Fj the tallness of the tall plant was visible or dominant. The dwarf- 

 ness of the tall plant was present in its make-up and might be inherited by its off- 

 spring but was invisible or recessive. 



He called them "formative elements" and assumed that they were units that 

 acted separately. With this correct interpretation Mendel laid the foundation 

 of modern genetics. 



Mendel's Principles 



The Law of Segregation. While both members of a given pair occur in an 

 individual only one of these is in a single egg or sperm. Thus, characters are 

 segregated. The nature of the members of each pair of opposite characters, 

 e.g., tall and dwarf in peas, or black and white in fowls, is not affected by the 

 other. The black that is inherited from hybrid gray parents proves to be as 

 black as if from pure black ones (Fig. 20.4). Characters are units which do 

 not blend or mix. 



The Law of Independent Assortment. Every character is inherited sep- 

 arately from every other character, in peas, the height of the plant from the 

 color of the flower. 



Dominance. When organisms, each with a pure-line for opposite characters, 

 are crossed, one character is either completely or incompletely dominant over 

 the other in the offspring (Figs. 20.3, 20.4); the other is completely or in- 

 completely recessive. Some characters are incompletely dominant, such as 

 the red of the red and white plants of four o'clocks that produce the pink 



