A.K.A. The Deep South
A highly derogatory reference to the supposed inbreeding that goes on in the backwaters of this area.
If your family tree doesn't fork (split), then that means your parents are blood relatives (e.g. they're brother and sister, or incestuous), and you probably have congenital abnormalities such as immune system deficiences, brain defects, etc.
Actually, inbreeding was a problem in the nobility of several countries in times long gone, including the British Royal Family.