Ok, so theoretically ...how many times / clutches would it take before you could say that a snake did NOT carry a certain gene?

Some of you know what I'm talking avout already I'm sure, but say I breed my enchi (that everyone believes is a pastel enchi) to a normal girl, got 0 pastels, and more that looked like him, and normals of course. Some could easily say that the ones looking like him were also pastel enchis. So how many pairings / clutches do you think would be sufficient to show he is not, and just bright? (Obviously, if he throws a pastel my brain debate is done)

Thanks