As far as I've seen in my own sims the most common command present in the genes of a reproducing bot is inc. According to the bot help file this command only increments a stored value in memory. Eventually that value will pass the reproduction value and the bot reproduces.
The reason why .repro is so rare and inc is so common as a reproduction method I think is because DB isn't totally random. It is somewhat seeded where you have a set pattern how the mutations will occur. There was a discussion here where we talked about making DB more random. I hope this can be implemented in DB3.
Actually, DB3 is almost definitely going to be more random.
Since I wrote the mutation code myself, I should know
In the mutation code I allowed for four input values that will determine how likely each kind of command/value is to be the result for the mutation.
In theory it should be one of the options in the mutation settings.
Also, I gave no preference at all to any one command over another, so .repro is just as likely as .setdx