Gonna wasn’t the first to Go Ssj2, But When he do’s stuff with videl, Then the gene would be passed down but it will get weaker every time. Have you see Dragon ball GT, Well In the 5th Generation, Goku Jr. He Transformed into SSJ but he doesn’t surpass that limit, And if he Does stuff with a human girl, Then The gene would end maybe in about the 7th to 10th Generation.