foreign keys constraints, depending on each other

I was just creating this little database for demonstrating the use of
foreign keys constraints.

I was about the create 3 tables, namely mother, father and child. Mother has
a foreign key pointing at father ( id ), and father has a foreign key
pointing at mother ( id ). Child has one pointer to mother ( id ) and one
pointer to father ( id ). How can I prevent the error message from occurring?

Ofcourse I see the problem here... just by taking away the references
keyword from the mother table takes away the problem completely.

---

DROP SEQUENCE mother_id_seq;
DROP SEQUENCE father_id_seq;
DROP SEQUENCE child_id_seq;

DROP TABLE mother;
DROP TABLE father;
DROP TABLE child;

CREATE TABLE mother (

id SERIAL,
fatherID INT4 NOT NULL REFERENCES father ( id ) ON DELETE CASCADE,

name TEXT,

UNIQUE ( fatherID )

);

CREATE TABLE father (

id SERIAL,
motherID INT4 NOT NULL REFERENCES mother ( id ) ON DELETE CASCADE,

name TEXT,

UNIQUE ( motherID )

);

CREATE TABLE child (

id SERIAL,
motherID INT4 NOT NULL REFERENCES mother ( id ) ON DELETE CASCADE,
fatherID INT4 NOT NULL REFERENCES father ( id ) ON DELETE CASCADE,

name TEXT

);

---

Thanks

Daniel Akerud