Final answer:
The phenomenon where two different true-breeding strains of sweet peas with white flowers produce F1 offspring with purple flowers is called incomplete dominance.
Step-by-step explanation:
The phenomenon where two different true-breeding strains of sweet peas with white flowers produce F1 offspring with purple flowers is called incomplete dominance. In this case, neither trait is completely dominant over the other, resulting in a blending of the flower color in the F1 generation. This is different from complete dominance, where one trait completely masks the other.