How many African-Americans can trace their roots and create a family tree which includes their ancestors in Africa?

In the series “Roots: The Saga of an American Family” the protagonist Kunta Kinte (captured by slave traders in 1767) wants to make sure that his descendants keep the memory of his parents who lived in the Gambia in West Africa. For generations, each of Kunta’s enslaved descendants passed down an oral history of Kunta’s experiences.

How realistic is this scenario? What about written accounts (created for example by slave owners)? Or is DNA analysis the only reliable option to find out where the ancestors of African-Americans lived?

From Wikipedia: Between 1650 and 1900, 10.24 million African slaves arrived in the Americas from the following regions in the following proportions:

* Senegambia (Senegal and The Gambia): 4.8%
* Upper Guinea (Guinea-Bissau, Guinea and Sierra Leone): 4.1%
* Windward Coast (Liberia and Cote d’ Ivoire): 1.8%
* Gold Coast (Ghana and east of Cote d’ Ivoire): 10.4%
* Bight of Benin (Togo, Benin and Nigeria west of the Niger Delta): 20.2%
* Bight of Biafra (Nigeria east of the Niger Delta, Cameroon, Equatorial Guinea and Gabon): 14.6%
* West Central Africa (Republic of Congo, Democratic Republic of Congo and Angola): 39.4%
* Southeastern Africa (Mozambique and Madagascar): 4.7%

http://en.wikipedia.org/wiki/Atlantic_slave_trade

Another question is: how many African-Americans are interested in finding out more about their roots?