Johannsen's Law is a principle in genetics that states that the effects of a gene can be separated into two components: the additive effects and the interaction effects. The additive effects refer to the contribution of individual alleles to a trait, while interaction effects arise from the combination of different alleles influencing the trait together. This law helps researchers understand how traits are inherited and expressed in organisms. By analyzing these components, scientists can better predict how traits will appear in future generations, aiding in fields like plant breeding and animal husbandry.