I'm a bit confused as to what is meant by "Bob, W because of X , Y because of Z".
If it's something like "Bob, is he W because of X genes, or Y because of Z genes?" I think your best way of showing it (other than in full like that) is "Bob -- W because of X or Y because of Z?" ie you can avoid repeating "Bob" and that's quite usual, but use a dash to hive off the clause and avoid needing a verb (a colon would do the same job but I'm a dashing person...) and no semi-colon at all, since it's not needed, and anyway should not be there with the "or". (Strictly, semi-colons shouldn't have link words like "or" and "and" but in a list an "and" often helps to show that's the last thing, so ignore the rule if it reads better. But here, with the "or" the semi-colon simply gets in the way, and it doesn't work on its own without the "or".)
If it's something like "Bob, is he W because of X genes, or Y because of Z genes?" I think your best way of showing it (other than in full like that) is "Bob -- W because of X or Y because of Z?" ie you can avoid repeating "Bob" and that's quite usual, but use a dash to hive off the clause and avoid needing a verb (a colon would do the same job but I'm a dashing person...) and no semi-colon at all, since it's not needed, and anyway should not be there with the "or". (Strictly, semi-colons shouldn't have link words like "or" and "and" but in a list an "and" often helps to show that's the last thing, so ignore the rule if it reads better. But here, with the "or" the semi-colon simply gets in the way, and it doesn't work on its own without the "or".)