Molecular analysis of a cell lineage.

Mating type interconversion has a precise lineage which serves to minimize the time taken for yeast cells to achieve the diploid state. The HO gene either encodes or regulates an endonuclease which initiates the interconversion process. Expression of this gene is switched on during the G1 phase of mother cells and not at all during the cell cycle of their daughters. This behaviour can explain what is known about the lineage.