The law of dominance does not occur universally. After Mendel several cases were recorded by scientists, where a clear deviation from law of dominance was seen. Such a deviation may be seen in the form of Incomplete dominance or Blending inheritance and Co-dominance.
Where $F_1$ hybrids exhibited a mixture or blending of character of two parents, the case is considered as that of incomplete dominance blending inheritance. It simply means that two genes of allelomorphic pair are not related as dominant or recessive, but each of them expresses itself partially. In the case of 4 o'clock plant, when plants with red flowers are crossed with plants having white flower, the $\mathrm{F}_1$ hybrids bear pink flower. When these pink flowers are self pollinated, they develop red, pink and white flowers in the ratio of $1: 2: 1$ respectively.