Two daughter cells receiving equal shares from the mother: that's the familiar picture of division. But what if one gets almost everything and the other just crumbs? Stochastic modeling shows that sometimes unequal division is more advantageous.

Each cell stores a protective reserve that helps it weather hardships. During division, this reserve is distributed between daughters. If the environment is shared by all cells, weak common influence favors equal division. But when common fluctuations are strong, it's better to give one daughter about eighty percent and the other twenty. The shift to such unequal division happens abruptly, like flipping a switch, not gradually like turning a dial.

The reason is that the shared environment affects everyone equally, but unequal inheritance reduces sensitivity to collective fluctuations. Even if the protective mechanism or memory of the reserve changes, the unequal branch persists, though its boundaries shift. Thus, a simple division rule explains why asymmetry in inheritance is found in living nature.