56 INBEEEDING AND OUTBREEDING 



result in the production of an intermediate. Often the 

 action of one factor dominates the action of the other, 

 either by masking it or by inhibiting its operation. When 

 this occurs the dominated character recedes from sight in 

 the Fi generation and the ratio in the F2 generation is 

 3 dominant : 1 recessive. But since only one-third of these 

 dominants breed true * and two-thirds behave as did the 

 Fi generation, the results are, therefore, comparable with 

 those illustrated by the wheat, and the phenomenon of 

 dominance is a mere detail. 



Mendel was not content with experiments in which 

 only one pair of differentiating characters was concerned. 

 He made crosses between varieties of the garden pea 

 which differed by two and by three allelomorphic pairs of 

 characters, and was rewarded for his perseverance by 

 discovering a second law, usually known as the Law of 

 Recombination. This laiv states that two or more allelo- 

 morphic pairs of factors may segregate independently 

 and may recomhi<ne in all the combinations possible gov- 

 erned by chance only. 



Though thousands of characters have been investi- 

 gated since MendePs time one cannot improve on the 

 original classic as illustrative material for explanation 

 of dihybrid heredity. 



With two pairs of characters he designated the factors 

 representing the dominant characters as A and B and the 

 factors representing the recessive characters as a and b. 

 The characters in the varieties crossed were as follows : 



Seed parent ( A form round Pollen Parent J a form wrinkled 



AB } B cotyledon yellow ab } b cotyledon green 



a When the factor for a character has been received from both parents 

 the organism is said to be homozygous for it; if it has been received from 

 only one parent the individual is heterozygous or hybrid for it. 



