[AccessD] Design question

Here's a design question I've never had to face before ... someone here
will have for sure.

I am in continuous development of a system which logs and analyses milk
production from a sheep farm.  Recently the client has requested an
(another) upgrade, this one to store some genetic data as well.  So now
we have 2 animal tables, tblEwes and tblSires.  

As part of the upgrade, a new Registration number is required for each
animal.  This must be unique across both tables.  How to implement?
Should I put in another table called, say, tblAnimalReg, with the
Registration number as PK?  And also store in this table the PK and
tablename of the animal it refers to.  Something strikes me as a bit
weak about this ... but I can't put my finger on it.  Ideas please.

Stephen Bond