Ref: DNA from the Beginning.org
In general, offspring appear to be a mixture of parental characteristics. However, Mendel found that this is not true for the pea plant traits that he chose to study. Pure-bred pea plants when crossed did not produce offspring with blended traits.
For example, one might expect that a cross between pure-bred green-seeded and pure-bred yellow-seeded pea plants to produce offspring with seeds of an intermediate green-yellow color. After all, color blending happens when paint is mixed together. However, Mendel found that this cross produced offspring with only one color — yellow. No intermediate blends were seen, and the green color seemed to have disappeared.