Drearyweather,
You did not read carefully what I wrote: Similarly, sometimes a person may slip into selfishness and use his free-will to his own harm or to the harm of others—but this does not make him a sinner because the ability to do the contrary (ability choose to commit virtuous act to any extent) too exists in him.
By saying: “The ability to do the contrary”what I meant is “ability to choose to commit virtuous act” which is also given in bracket.
If sin is an inherited tendency as you believe, how can people at times choose to commit virtuous act? Evidence is that anybody can choose to sin or choose to do virtuous act, or choose to mix them in any proportion he likes. This is why we find people acting/reacting in various ways in same situation.
Sin is not hereditarily passed, but the path of sin is as follows:
From beliefs, attitudes are formed
From attitudes, thoughts are formed
From thoughts, action is produced
From repeated action, tendency, habit, destiny …etc are produced.
That means, it is easier to change your course in the earlier parts of the process.