1) As every organism which reproduces sexually has 2 traits in which 1 is Dominant while the other is Recessive. 
2) In the Progeny only the dominant traits are expressed while the recessive is existing but hidden. 
3) According to the Mendel's experiment, he cross pollinated between the tall and the short plants and in the progeny he found that all the plants are tall and no mid way plants. 
4) Hence it can be concluded that in the progeny the 2 traits exist but may be expressed.

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Yours, Jahnavi