How does comparative genomics aid in the search for factors that contribute to complex human conditions?

After the examination of the human genome became possible, scientists could compare the genes of "normal" individuals with the genes of individuals with disorders. Once scientists know where the gene is affected, they can determine the factors that could have impacted that specific portion of the gene. They can examine the genes of family members of each individual to see if conditions are determinedly hereditary or if environmental factors may play a role.

I’d say a : water molecules