Finaly we decided to change the cardinalities. It means a stallion could have or not a father, in fact regarding the context it is logical. Hoiwever this exercise and question could be useful for others that met, meet, or will met this problem in Microsoft Access. It is useful to keep this post in your database.
I want to say thx very much to you all for your help, finally U did help us to see the reality of the problem.
Regards.
