Two Pairs of Genes in Two Pairs of Autosomes 131 



into a ratio of 1 dominant : 2 intermediate : 1 recessive. If a 

 plant is segregating independently for two pairs of alleles and 

 if dominance is incomplete in both, the F2 phenotypic dihybrid 

 ratio is the same as the genotypic. In cotton, when a plant with 

 brown lint (WW) is crossed with a plant that has white lint 



n 



m 



7y^ 



d.48 d.68 d.65 

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d.68 

 Stroke 



56 56 55 49 56 d.41 50 45 44 40 



49 56 



(^Jli 



d.41 50 

 Stroke 



45 44 



26 16 28 23 29 26 10 10 16 13 



m and ^p Hemangioma (Blood Tumor) 



/' Long Second Toe (Longer Than First) 



Fig. 43. A dihybrid pedigree in human beings. (From Beers and Clark 

 in the Journal of Heredity.) 



(ww), the Fi is cream (Ww) and the F2 segregates into a ratio 

 of 1 brown : 2 cream : 1 white. When plants with red-spotted 

 leaves (SS) are crossed with plants that have no spots on the 

 leaves (ss), the Fi (Ss) has spots but they are of weaker in- 

 tensity than the spots on the red-spotted parent. The F2 segre- 

 gates into a ratio of 1 deep spots : 2 pale spots : 1 no spots. If 

 plants which are spotted and have brown lint (SS WW) are 

 crossed with plants that have no spots and white lint (ss ww) , 

 the Fi has intermediate spotting and cream-colored lint, and 

 the F2 segregates into: 



