392 RE.\DINGS IN EVOLUTION, GENETICS, AND EUGENICS 



hybrid progeny would all be tall, smooth, yellow, since these three 

 characters are dominant. Inbreeding the hybrids gives the following 

 result in the F2 generation: 27 tall smooth yellow, 9 tall smooth green. 



Dwarf Wrinkled 

 Parent 



Gametes 



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(w) (w 



T) {^D 

 S)'(S 



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Fig. 69. — Diagram illustrating dihybrid ratio. Upper part shows how original 

 parents were crossed to give Fj hybrid; lower part shows Fi hybrid producing 

 four kinds of gametes; chance matings among these gametes, when Fi is inbred, 

 result as indicated in the large set of squares and explains the 9:3:3:1 ratio in 

 the F2 generation. {From Coulter atid Coulter.) 



9 tall wrinkled yellow, 9 dwarf smooth yellow, 3 tall wrinkled green, 

 3 dwarf smooth green, 3 dwarf wrinkled yellow, i dwarf wrinkled 

 green. The trihybrid ratio therefore is 27:9:9:9:3:3:3:1. This 

 involves 64 individuals and 8 phenotypes. 



