A single change in the
DNA of the hemoglobin gene will cause

A “mutation” changes the instructions of a gene, which can also change the protein. Sometimes this change makes it so the protein can't do its job as well. And this is what happens with sickle cell anemia. A mutation in the hemoglobin gene changes the hemoglobin protein in a way that causes sickle cell anemia.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.

affects the water bodies: excessive use of fertilizers causes eutrophication. the fertilizers which are used on plants affect the quality of land on which it is used, further causing erosion of soil. the fertilizers used on plants contain harmful chemical fumes which affect the environment.