where k 3 is the average number of times one type of cells is generated. cn should be the number of cell types or the number of linkage groups. In fact, in the organs or tissues of multicellular organisms, many cells are linked, causing repetitive pattern in the organ or tissue. So the numbers of cell types cn in the formula should be replaced by the number of linkage groups. There is no regulatory information needed for genes or cells inside a linkage group. However because the numbers of linkage groups in organisms are still unknown, we calculated I 3 based on the number of cell types.