Mendel's parental pea plants always bred true because both gametes they produced:

1. A true-breeding line gives offspring identical to itself generation after generation. 2. That happens only when there is no hidden alternative allele to reappear. 3. Mendel's parental pea plants always bred true because both of the gametes produced carried the same trait. 4. A plant carrying two different alleles would segregate them and produce mixed offspring. 5. That is exactly what the F1 hybrids did in the next generation. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 12 'Mendel's Experiments and Heredity', sections 12.1-12.3_