Joint ownership in patents

Are you talking about being a joint owner or a joint inventor? To be a joint inventor, you must have contributed something to the invention, and not necessarily an equal percentage of the overall invention. As for being a joint owner, that is basically a contract question - for example, did you invest in the development of the invention, and therefore in return for your investment, you have a right to be a part owner of the patent if that was part of your original contract upon which your investment was made. Alternatively, did the inventor work for you, and therefore, per his employment, he is required to assign part or all of his rights to his invention to you.