Mendel’s laws of genetics and their modern-day refinements explain almost all physical variations occurring within species. Mendel discovered that genes (units of heredity) are merely reshuffled from one generation to another. Different combinations are formed, not different genes. The different combinations produce many variations within each kind of life, as in the dog family. [See Figure 3 on page 4.] A logical consequence of Mendel’s laws is that there are limits to such variation.a Breeding experimentsb and common observationsc also confirm these boundaries.