Why 'another' instead of 'the other'?

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.

The Ascent of Humanity

• I think "the other" would in fact be better here. Commented Mar 16, 2021 at 1:55
• I can only agree with @EthanBolker that this appears to be an unfortunate word choice.
– Mary
Commented Mar 16, 2021 at 1:56

Q. 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?

Yes I believe you are correct. It is a different gene

This not my specialist subject but I believe another is being used as a pronoun and the Antecedent is “gene that only differs by a single base pair

If two genes differ by only a single base pair, then a single point mutation could convert one into a differentgene that only differs by a single base pair”.

My reasoning for this is that the text specifically states "a single point mutation" (not just any mutation), which I am guessing would lead to a variation of a "single base pair"

However later in the text another is not used as a pronoun. "or one after another" could be replaced by "or one after an other"

Q. Why 'another' instead of 'the other'?

Another can be used as a pronoun whilst I find no reference to other being a pronoun in this context.

another determiner, pronoun (DIFFERENT) a different person or thing:

Ref CED Another

Ref Grammar Revolution What is a pronoun