How does Mendel's work demonstrate support for the concepts of dominant and recessive alleles?

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Mendel shows that dominant alleles always took over, while sometimes in the F2 generation recessive traits would come back. As you can see, f rom the proportions of phenotypes in the offspring. Then, if given that the genes would be equal you would get 1/4 A, 1/4 B and 1/2 AB; but you often get 1/4 A and 3/4 B.