law of dominance means:In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”
Here pure means monohybrid whereas contrasting traits mean any two form a particular trait. For example, say height- Pea plant could be of two possible heights : either tall or short.
So, Mendel takes two pea plants: One short pea plant (thus havinggenes “tt” for expressing height) and one tall pea plant ( Having agenes “TT” which express or determine its height.)
Then he bred them by pollinating the flower of one monohybrid short pea plant with pollen of monohybrid tall pea plant.However, when he planted these seeds after harvesting, he found that all of the offspring plants were tall.
So, if there exists two contrasting traits, one of the traits will always suppress the other, thereby expressing itself. T suppresses t, thus making the offspring plant tall.
Such a trait is known as a Dominating trait. The suppressed trait is known as Recessive trait. Also, therecessive trait freely expresses itself in the absence of the dominant state. And this is what Mendel’s Law of Dominance is all about.