When two plants, both tall, were crossed, they produce some offspring that were dwarves. assuming that the height of the plant is controlled by a single gene and the allele for the tall plant is dominant, what is the genotype of the parents

The genotypes of the parents would have to be Tt and Tt. It could not be Tt and tt because both plants are tall. It could not be TT and Tt because then all offspring would be tall. The Tt cross would give offspring of TT, Tt, Tt, and tt, which fits all the requirements. 

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