As David as pointed out the children of mixed white and indian heritage were called "halfbreeds" and still considered as being Indian. These, what we would call, racist attitudes, were held all the way into our present day generation if any of us will admit that.

For example, the children of white and Japanese heritage were still considered as Japanese during World War II and we gathered them up and sent to interment camps. To carry it even farther during the Korean War and the Vietnam war you have the abandonment of the children father by American soldiers. Who were considered to be Korean or Vietnamese even though the father was White, African American of even Oriental Americans. And I am sure there are even some cases of this in Iraq. Although I would imagine that there are very few cases due to culture clash.

These attitudes were NOT unique to just the southern people. They were universal throughout all the peoples of the United States. My maternal ancestors for example were very clanish. Which was not uncommon in European societies and cultures. They would only marry within their own culture. For 300 years since arriving in America my maternal ancestor maintain a 100% germanic heritage.

So in answer to Chase question as to Why the white parentage of Fredrick Douglas was ignored? It was because that was the culture of the time to do so, not in just southern cultures but in almost all cultures.