Answer:
See the answer below
Explanation:
The explanation I would give to the phenomenon is that the round trait in peas completely covers or nullify the wrinkled trait when they are together. In order words, whenever a single the allele of both trait come together to form a new trait, the round trait allele dominates and mask the expression of the allele of the wrinkled trait.
This is what Mendel explained and dominance/recessive effects. The round trait was dominant over the wrinkled trait while the wrinkled trait was said to be recessive and can only be expressed when alone.