If there are two genes that are different by only a single base pair, and a single point mutation happens, why could one be converted into another, not the other? Does the author mean a new, different third gene?
Generally speaking, a single mutation in an existing gene cannot produce a new gene with a different function. If two genes differ by only a single base pair, then a single point mutation could convert one into another. Usually, however, these two genes will be regarded merely as variants of the same gene, and will have an identical function unless the mutation is at some critical spot that renders the second gene non-functional. The same goes for other basic alterations, such as the deletion or repetition of an existing sequence. To get a gene with an entirely new function usually requires many, many alterations, a concatenation of several unlikely steps — a series of just the right mutations happening either all at once, or one after another. Unlikelihood multiplies into impossibility. If I guess the next card you draw from the deck, you’d be impressed but not amazed. If I guessed ten in a row, you’d suspect a trick because that would happen only about once every fifty quadrillion trials.
