The principle of dominance is a genetic law that states that the offspring of individuals with contrasting traits will only express the dominant trait. The discovery was made by Gregor Mendel and thus, the law is also called "Mendel's Law."
Mendel observed that while only the dominant trait showed in the second generation offspring, the invisible, or recessive trait, reappeared in the third generation. However, Mendel only observed one type of dominance, now called complete dominance. There are several other kinds of dominance, including incomplete dominance, codominance and pseudodominance.
Other phenomena also impact gene expression. These factors include variable penetrance, variable expressivity, and epistasis.