Why did the mistress of the plantation slap Nanny ?

The mistress of the plantation slapped Nanny many times because the baby looked partially white with its blonde hair and gray eyes. The mistress of the plantation knew that the master had been sleeping with Nanny and threatened to whip her until she bled to death and sell the baby into slavery when it was a month old. This excerpt was taken right from the GradeSaver Site. Check the link out below.