In other words: Since interracial marriage was illegal in most states, one of your ancestors may have been the result of an illicit affair between a black person and a white person, with the resulting child being raised with the white birth parent and a white adoptive parent.
Also, we agree with you that based on the DNA testing you have done thus far, it seems likely that your African-American ancestry probably came from your maternal grandfather’s mother’s side of your family. However, you will want to double-check that your mother and the third cousin you mentioned share both of their second-great-grandparents, not just the second-great-grandfather.
If so, says, Moore, “My advice is for him to track down a direct maternal-line descendant of his maternal grandfather’s mother, such as a child of his great-aunt or a child of his great-aunt’s daughter, and ask him or her to take a mitochondrial DNA test. Mitochondrial DNA (which should not be confused with X-chromosome DNA) is passed from mother to child. Both males and females inherit it from their mother, but only females pass it on to their children. If the mitochondrial DNA comes back with an African haplogroup, then you know specifically which line he should be focused on. If it doesn’t, then he can rule every one in that direct maternal line out. He can repeat this for the various ancestral lines until he hits on one that has an African haplogroup.”
Moore also suggests using a testing service to see if anyone else in its database matches your mother on one of the segments of autosomal DNA that is predicted to be of African origin. 23andMe is one of the companies providing this service. She adds, “If he can find additional matches that cluster on that segment, then he should try to find a common ancestor between those matches.” There is a very good chance that you would share that ancestral line with any individuals who match.
Armed with the new information you uncover from additional DNA testing, you can then refocus your record search for your elusive African-American ancestor.
As Moore stated, sometimes documents can be misleading. Perhaps one of your ancestors was raised by people who weren’t his or her birth parents. Another possibility is that your ancestor was born outside of a marriage, as interracial marriage was once illegal in most states. It may be difficult to determine what exactly happened, but a thorough search of different documents may help you find more possibilities and rule out others.
Researching an Unrecorded Adoption
Let’s first look at the possibility that a white person or couple adopted one of your ancestors. The earliest adoption legislation was created in the 1850s. Even then, some places were much slower to implement any formal laws on the guardianship of children. The adoption process, as we know it today, didn’t really start until the early to mid-20th century. Before this time, adoptions were less-formal arrangements by which a family would assume the responsibility of a child who was not their own. As such, if your ancestor was raised by someone who was not his or her biological parents, there probably isn’t a record of it in any court.
Although there may not be an adoption record in the court records, you still might want to consider searching for probate and will documents for your ancestors in your direct line who you believe have the African-American history. Probate documents deal with the division of a person’s estate after his or her death. They can sometimes shed light on family dynamics that aren’t apparent in census and vital records. If your ancestor owned any real estate or had a personal estate of value (this information is often listed in census records), try searching for them in the probate court where he or she died.
If you are able to find a will or probate record for an ancestor in the line you are researching, you will want to look at it closely to see if anything doesn’t make sense or seems odd. For example, is there any child still living, but not listed in the will? Is one child given a much smaller portion of the estate than the others? The FamilySearch research wiki can help direct you on how to find probate court records in the area and time period you are researching by typing in the place you are researching along with the words “probate records.”