LEE BARKER WALTON 81 



detail here because of their bearing on mutation, con- 

 sisted primarily in the crossing of tall and dwarf peas, 

 with the result that the first filial (Fi) or hybrid genera- 

 tion consisted entirely of tall plants. When, however, 

 seeds from these plants were sown the ratio of tall to dwarf 

 plants became 3 to 1 in the second (F2) hybrid generation, 

 a result explained by the theory of dominant and recessive 

 characters on the basis that there are certain determiners 

 of unit characters in the germplasm which dominate over 

 others during the development of the somatoplasm or 

 body of the individual in the higher forms of life. More 

 recently the presence and absence theory has been applied 

 in interpreting the results. In a manner similar to the 

 preceding when smooth yellow peas were crossed with 

 wrinkled green peas the first hybrid generation consisted 

 of smooth yellow forms inasmuch as the character smooth 

 and the character yellow were dominant over the character 

 wrinkled and the character green, and the crosses were 

 known as dihybrids, inasmuch as they differed in respect 

 to two characters. In the second hybrid generation the 

 resultant ratio was 1 5 to 1 pure recessive, /'. e., wrinkled 

 green, although the fifteen consisted of smooth yellow, 

 smooth green, and wrinkled yellow in the proportion of 

 9:3:3. In the same way trihybrids have the ratio 63 to 

 1 pure recessive while any polyhybrid differing in ^^n" 

 characters which mendelize in the usual manner will give 

 an expected ratio of 4^? — 1 to 1 pure recessive, which will 

 become apparent only through the breeding of large num- 

 bers of individuals. 



While the preceding summary represents the normal 

 results in connection with the segregation of unit char- 

 acters, studies of the past few years have demonstrated 

 that many interesting relationships may occur between 

 the factors governing the production of characters. For 

 example, it has been found that two or more determiners 

 are often present either of which will produce the given 

 character as Nillson-Ehle demonstrated in hybrids of 

 brown and white chaffed wheat, while on the other hand 

 two or more determiners acting together may be necessary 

 to bring about an effect. Such a condition exists, as 



