Answer:
It's impossible to predict the phenotype of the offspring by only observing the parents because DNA from their grandparents can affect the offspring as well.
Step-by-step explanation:
DNA is combined from the parents to create offspring. When that offspring reproduces their children not only possess DNA from their parents but from their grandparents as well. Mixing together two separate DNA's from two different family trees can result in rare genetic mutations which results in the offspring looking different from their parents but showing resemblance to their grandparents. This is why you have to look at the phenotypes of more then just the two parents because there are more possibilities, including what their grandparents looked like.