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The parents are two purebred plants: tall(TT) and dwarf(tt).

Both of them contribute one gamete each with one allele and they fuse to form the first(F1) generation. The F1 generation plants are all tall(Tt).

Now the F1 generation plants contribute two types of gamete each: T and t from both the plants. Now crossing over happens as shown in figure and three types of plants are produced in the second(F2) generation. The plants are tall(TT), tall(Tt) and dwarf(tt).
The phenotypic ratio in the F2 generation is 
tall : dwarf = 3:1
The genotypic ratio is
TT:Tt:tt = 1:2:1
1 5 1
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