If you really want to complicate things, you could say there are epigenetic components or environmental stressors (think something like allergies) that can take a carrier and make them a practitioner.
And if you want to make things interesting as well as complicated (and a little recursive) some of those stressors could be exposure to magic
-Terry
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"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy
And if you want to make things interesting as well as complicated (and a little recursive) some of those stressors could be exposure to magic
-Terry
-----
"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy