Checking descendants of a family tree in Prolog - recursion

Question

How can I recursively check if X is the descendent of Y, using a recursive method descendent_of? The fact base consists of males and females, and father_of/mother_of statements.
/* Facts */
male(roy).
male(lee).
...
female(joy).
female(ana).
...
/* Rules */
grandmother_of(X, Z) :-
mother_of(X, Y),
(mother_of(Y, Z);
father_of(Y, Z)).
grandfather_of(X, Z) :-
father_of(X, Y),
(mother_of(Y, Z);
father_of(Y, Z)).
parent_of(X,Y) :-
mother_of(X,Y);father_of(X,Y).
descendent_of(X,Y) :-
*/ Recursive method here */
How do I set up the stopping condition? Also, how does recursion work in prolog if functions only return boolean values?
Any comments/suggestions are greatly appreciated.

Answer 1

If you look at descendant_of(X,Y) it should be true if X is descendant of Y. The truth value is than reursivly defined as follows:
I do here a little trick to achieve consistency with parent_of and introduce the predicate ancestor_of/2 as
descendent_of(X,Y) :- ancestor_of(Y,X)
Know X is ancestor of Y if either
X is parent of Y or
X is parent of Z and Z is descendant of Y
(1) is the base case of the recursion and (2) is the recursive definition.
Hint: You could also redefine the recursive case as: Z is descendant of Y and X is parent of Z (Keyword tail-recursion)

Answer 2

I've extended your facts to give some working data.
/* Facts */
male(roy).
male(lee).
female(joy).
female(ana).
parent_of(roy,joy).
parent_of(joy,ana).
parent_of(lee,ana).
Now we can build some rules. Let's start with grand_parent_of/2.
grand_parent_of(X,Z) :-
parent_of(X,Y),
parent_of(Y,Z).
This keeps the rules for grand_father_of/2 and grand_mother_of/2 rather simple.
grand_father_of(X,Y) :-
male(X),
grand_parent_of(X,Y).
grand_mother_of(X,Y) :-
female(X),
grand_parent_of(X,Y).
Now, every predicate so far the ancestor has been on the left and the descendent on the right, so to keep consistency, let's define ancestor_of/2 first, rather than descendent_of/2.
ancestor_of(X,Y) :-
parent_of(X,Y).
ancestor_of(X,Y) :-
parent_of(X,Z),
ancestor_of(Z,Y).
Note that the second predicate is recursive.
Now we can easily define descendent_of/2 simply in terms of ancestor_of/2.
descendent_of(X,Y) :-
ancestor_of(Y,X).
Here are the results of running the following query and asking prolog to keep giving me results.
?- descendent_of(X,Y).
X = joy,
Y = roy ;
X = ana,
Y = joy ;
X = ana,
Y = lee ;
X = ana,
Y = roy ;
false.