Phenomena of Inheritance 



91 



two contrasting characters in the parents the offspring arc dihy 

 brids, when three, trihybrids, and when the parents differ in more 

 than three characters the offspring are called poly hybrids. Th< 

 are certainly few cases in which parents actually differ in only a 

 single character, but since each contrasting character may be 

 dealt with separately, as if it were the only one, and since the 

 number of types of offspring increases greatly when more than 

 one or two characters are considered at the same time, it is cus- 

 tomary to deal simultaneously with only one or two characters 

 of hybrids, even though the parents may have differed in many 

 characters. 



Dihybrids. — When two or more contrasting characters of the 

 parents are followed to the F 2 generation many permutations of 

 these characters occur, thus giving rise to a larger number of 



R w 



p 



&^JB 



(o ojf o o To 0Y0 O) F, 



RlW) RlW) RlW) R(W) 



\o* B w^ 



Fig. 30. Monohybrid Diagram Showing Results of Crossing Round- 

 (R) seeded with Wrinkled- (IV) seeded Peas. Large circles represent 

 zygotes, small ones, or single letters, gametes. In F t all individuals are 

 round but contain round and wrinkled gametes. In F, the S gametes 

 are placed above the square, the 9 ones to the left, and the possible com- 

 binations of $ and 9 gametes are shown in the small squares, the relative 

 numbers of different types being 1 RR : 2 R(\V) : 1 WW, 



