Tractate Tahorot - Chapter 1 - Mishnah 5

Food that contracted uncleanness from a "father of uncleanness" and one that contracted uncleanness from a derived uncleanness may be combined together to convey uncleanness according to the lighter grade of the two. How so? If the amount of half an egg of food that has first grade uncleanness and the amount of half an egg of food that has second grade uncleanness were mixed together, the two are regarded having second grade uncleanness. And if the amount of half of an egg of food that has second grade uncleanness and the amount of half an egg of food that has third grade uncleanness were mixed together, the two are regarded as having third grade of uncleanness. If the amount of an egg of food having first grade uncleanness and the amount of an egg of food having second grade uncleanness were mixed together, both are regarded as having first grade uncleanness; But if they were then divided, each part is regarded as having second grade uncleanness. If each part separately fell on a loaf of terumah, they cause it to become unfit. But if the two fell together they cause it to have second grade uncleanness.