system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] Are the Doctor's future regenerations built into his genome?

If we fully sequenced the Doctor's DNA (if he even has it), could we predict the appearance/personality of his next regeneration? Do we even have a biological explanation for regeneration?
gpt
The change is dependant on subconscious and situation. You can control regeneration with practice and meditation (as mentioned with Romana, and as Master statedly turns into a younger version to reflect Tennant's Doctor).

Doctor could have some control over regeneration, but the way he handles these crisises (acute immediate death seconds away) doesn't leave much time for thought. It might also be that Doctor isn't particularly focused (as Romana implies) and rather relies on subconscious. 

Which is why #10 spoke like Rose and #9, #10, #11 were all younger than the previous incarnation. Bad Wolf indicates this to be a wish to escape the guilt and life-experiences of the War Doctor.  As #11 succeeds in getting rid of the guilt, #12 is again older (closer to the "long term average") and not looking like a doctorate-student (like Matt Smith).