Randomly. Any finite number of items (4 genome nucleotides G,A,T, and C) assembled in a finite sequence (one chromosome) has a finite number of possibilities. An unimaginably large number of possibilities, but still finite. Genes mutate randomly (lightning strikes, radiation events, etc)so any possible sequence can happen naturally, mathematically speaking. So you can't say that it would "never" occur, it's always technically possible. It's just that you might have to wait a very long time for it.
Now that line of thinking has been beat into the ground..... It was actually supposed to be lighthearted and funny.
Hah, sorry, engineers are thorough and lose what little sense of humor we have in our quest to make sure we fully understand the topic and conversation.