Hi there!

The law of dominance is not universal because phenomena like incomplete dominance and codominance are exceptions to it. 

According to law of dominance, out of the two parental traits, only one is able to completely express itself over the other in the F1 generation. Thus, the F1 progeny resembles the dominant trait while the recessive trait remains hidden or masked.

But in incomplete dominance, the F1-progeny resemebles neither of the two parental traits, rather the trait expressed is an intermediate between the two parental traits.

While in codominance, the F1-progeny resembles both the parental traits as both the parental traits are equally expressed.

Thus, in both these cases, we can say that the Law of dominance is violated. Hence, it is not a universal law.

Thanks