Mich. Comp. Laws § 330.1214: Board; county representation.

Sec. 214.

When a single county establishes a board, all board members shall be representatives of that county. When a combination of counties establishes a board, unless otherwise agreed to by each of the participating counties, the board memberships shall be divided among the counties in proportion to each county's population, except that each county is entitled to at least 1 board membership.