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THE POPULAR SCIENCE MONTHLY 



But 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 types of individuals than 

 when a single pair of characters is concerned. When there is only 

 one pair of contrasting characters there are usually but two types of 

 offspring apparent in the F 2 generation, viz., dominants and recessives 

 in the ratio of 3 : 1 (Fig. 53) ; where there are two pairs of contrasting 



RlW) RIW) R(W) RIW) 

 \c? R W 



H 



Fig. 53. Monohybrid Diagram Showing Results of Crossing Round (R)- 

 Seeded with Wrinkled (W) Seeded Peas. Large circles represent zygotes, small 

 ones, or single letters, gametes. In F x all individuals are round but contain round and 

 wrinkled gametes. In F 2 the g gametes are placed above the square, the J ones to the 

 left, and the possible combinations of g and J gametes are shown in the small squares, 

 the relative numbers of different types being 1 RR :2 R(W) :1 WW. 



Fig. 54. Diiiybrid Diagram Showing Results of Crossing Peas Having Yel- 

 low-round (YR) Seeds with Others Having Green-wrinkled (GR) Ones. Four 

 types of germ cells are formed by such a hybrid, viz., YR, YW, GR, GW, and the 16 

 possible combinations (genotypes) of these g and <j> gametes are shown in the small 

 squares. Since recessive characters do not appear when mated with dominant ones 

 these 16 genotypes produce 4 phenotypes in the following relative numbers : 

 9YR : 3YW : 3GR : 1GW. There is 1 pure dominant (upper left corner), 1 pure reces- 

 sive (lower right corner), 4 homozygotes in diagonal line between these corners and 

 12 heterozygotes. 



characters in the parents there are four types of offspring in the F 2 

 generation in the ratio of (3 : 1) 2 = 9 : 3 : 3 : 1 ; when there are three 

 pairs of contrasting characters in the parents there are eight types of 

 offspring apparent in the F 2 generation in the proportions of (3: 1) 3 = 

 27:9:9:9:3:3:3:1, etc. Thus when Mendel crossed a variety of peas 

 bearing round and yellow seeds with another variety having wrinkled 

 and green seeds all the offspring of the ¥ 1 generation bore round and 

 yellow seeds, round being dominant to wrinkled, and yellow to green. 



