public class RootSection<T, TChild> : Section<T>
    where T : Section<T>
    where TChild : Section<TChild>
{
    public List<TChild> ChildSection { get; } // duplicate
}

public class MiddleSection<T, TChild, TParent> : Section<T>
    where T : Section<T>
    where TChild : Section<TChild>
    where TParent : Section<TParent>
{
    public List<TChild> ChildSection { get; } // duplicate
    public TParent Parent { get; } // duplicate
}

public class BottomSection<T, TParent> : Section<T>
    where T : Section<T>
    where TParent : Section<TParent>
{
    public TParent Parent { get; } // duplicate
}

public class Section<T>
    where T : Section<T>
{
    List<T> AdjacentSections { get; }
}

public class Map : RootSection<Map, Biome> { } // (T, TChild)
public class Biome : MiddleSection<Biome, Landform, Map> { } // (T, TChild, TParent)
public class Landform : MiddleSection<Landform, Tile, Biome> { } // (T, TChild, TParent)
public class Tile : BottomSection<Tile, Landform> { } // (T, TParent)