Ref: DNA from the Beginning.org
Mendel believed that genes behave like atoms that compose a pure substance. Genes can combine in various ways but always maintain their distinct identities. For example, in a cross between two pure-bred parents with different traits like seed color, the hybrid offspring would have both the gene alternates for green and yellow seed color.
Why then do offspring from such a cross have only yellow seeds? Mendel proposed that although both gene alternates are present, there is no blending of color because the gene alternate for yellow is “dominant” over the gene alternate for green. The dominant trait is seen whenever a single copy of its gene is inherited. When he crossed the hybrid offspring, green seeds reappeared in the next generation. Mendel reasoned that the “recessive” green trait is shown only when a copy of the recessive gene form is inherited from each parent.