How and why does the mood change throughout?

The mood changes, as the baby's heritage comes to light. At the start of the story, the birth of Desiree and Armand's child is a reason for celebration.... a time of joy. As the story progresses, and the characters begin to question the baby's ancestry..... social status, and with this questioning the mood becomes tentative, unsure, and disturbed. Sadly, those who experience this alarm are incorrect in their assumptions.