[Free Registration/Login may be required to access all resource tools.] Sometimes understanding how common a gene is within a population is necessary. Or, more specifically, you may want to know how common a certain form of that gene is within the population, such as a recessive form. This can be done using the Hardy-Weinberg model, but it can only be done if the frequencies of the genes are not changing. Learn more about hardy-weinberg theorem in this learning module produced by CK-12.