The parent clustering matrix specifies at each genome position the results
of a parent clustering into ancestral groups. The matrix rows
represent the position and the columns correspond to each parent. For example,
if we have at the ith row (1, 2, 3, 2, 1), this means that parents 1
and 5 are in the same group, that 2 and 4 are in another one and that the
third parent was assigned to any group.