Sometimes genes, and sometimes a lot of singing, experience and practice. I'll say it's a mixture of both.I view it like learning a new sport.. Some people are naturally talented and pick everything up right away and be the next MVP. Other people eventually become strong or outstanding players but start off weak and having to learn a lot of the stuff from stratch. Then there are those who just naturally... suck...
Then again, it's been proven if you have the will (and the pockets full of money) to be a singer and willing to take years of singing lessons on a weekly basis, people with little skill over the voice can drastically improve.
That's just my take on singing... =\