As a PhD in genetics, what would you tell someone about DNA testing that didn’t show obvious Cherokee DNA from a g-g-g grandmother that family legend said was at least part Cherokee? Does the DNA test disprove that particular family legend?
Actually, my Ph.D. is in biochemistry and molecular biology, not genetics. I study the behavior of biological molecules, although I do understand genetics.
Anyway, the answer to your question starts with the fact that each child receives roughly half of the DNA of each parent. So, the contribution of any particular ancestor in the family tree becomes very small within a few generations. You have 50% of your parent’s DNA, 25% of your grandparent’s, 12.5% of your g-grandparent’s, then 6.25 (gg), 3.125 (ggg), etc.
The other factor is that DNA “recombines” in the germ cells. During recombination, each chromosome that you received from your mother pairs up with the corresponding chromosome from your father and they swap sections. Like this:
aagactaggacctc (maternal chromosome)
AAGACCAGGACTTC (paternal chromosome)
AAgacCAGGAcctc and aaGACtaggaCTTC (germ cell chromosomes)
So, let’s say that the “cctc” is a Cherokee gene marker and “CTTC” is a Caucasian marker. So if your child ends up with the “CTTC” chromosome, he or she has completely lost that marker. Please note that I used lowercase and uppercase to denote which chromosome came from which parent; this usage has no genetic significance.
I made these “chromosomes” very small to illustrate the point. In reality, we are talking about chromosomes that swap thousands or millions of letters across their length of tens to hundreds of millions of letters. (Each letter is a nucleotide.) A genetic marker can be as small as a few dozen letters. These markers make up only a tiny fraction of the DNA: if you take any two people at random, they are 99.5% identical genetically. So, we are only looking at that 0.5% of DNA to make ethnic comparisons.
Through recombination, it is possible for genetic markers representative of a specific ethnic background to completely disappear. In my case, I can find German immigrants in my family tree and I have a German family name—but my ethnic genetic profile does not show German ancestry at all. Primarily, it shows British, Irish, Scandinavian, Iberian, and traces of Bantu, Indian (not native American), and the Caucasus.
One last factor is that a genetic marker is not necessarily unique to a certain ethnic background. The determinations of ethnicity via genetics are made using statistical methods. So, going back to my example above, with “CCTC” and “CTTC” being the ethnic marker, we would determine the prevalence of each marker in each population. For example, we can determine that 90% of Cherokees have the “CCTC” marker, and 75% of Caucasians have the “CTTC” marker. So even if you have the CTTC marker, it is not proof that you have no Cherokee ancestors. When the sites like Ancestry.com, etc., look at markers, they actually derive a probability of your ancestry using statistical methods. They look at each marker’s prevalence in various populations, and how many of each marker you have. For example, if you have 10 markers that each occur in 50% and 20 markers that occur in fewer than 10% of Iberian people, they will use that data to calculate the likelihood that you have Iberian ancestry.
One last little complication is the fact that, although you share 50% of your DNA with each parent, you share variable amounts of DNA with your siblings. It is possible (but not likely) that you share no or all of your DNA with your sibling. So your ethnic profile might be different from your sibling’s—i.e. your sibling might show Cherokee ethnicity while you do not.
Yeah, I know—this is a very long and complicated answer to your questions. I hope it is all clear; feel free to ask for further explanation on any point.
I noticed on the preview that my chromosome illustrations don’t line up... they would look better in a fixed font, but I don’t know how to make html show a fixed font. The reply window has a fixed font.
When I was five my mother showed me a picture of my great-grandmother (from my mother's side of the family) who was very ill when the picture was taken and I was barely three months old. She made a comment at the time that she looked like an Indian squaw. Being five I thought that she was an Indian squaw and I thought that I had some Indian blood in me. A couple years ago when my daughter and I began doing our family tree I asked my mom about the picture and she told me that she was not Indian but Slovakian.
I have to admit I liked the thought of having Indian blood.