The Genetic Genealogist

This week I had a terrific email conversation with Jasia from The Creative Gene about genetic genealogy. She left a comment on a recent post, Discovering My Maternal Roots, which asked:

“I’m a complete neophyte about DNA for genealogy. I’m wondering if there is any reason to test myself, and my mother. Since the mtDNA seems to trace the maternal line… is it enough to test just one of us or is there something to be learned by testing both of us?”

This is a great question, one that many people who are new to genetic genealogy ask. Understanding how mtDNA and Y-DNA are inherited is one of the most challenging aspects of genetic genealogy. I always think of them as mirror images; if you chart your family tree, the Y-DNA travels down the far left line (from your father’s father’s fathers’ father…) while the mtDNA follows the far right line (from your mother’s mother’s mother’s mother…). Here is my response to her comment:

“Thanks for stopping by! In answer to your question, there is usually no reason to test both your mtDNA and your mother’s mtDNA, since it is the same. Only in VERY rare occasions will a detectable mutation arise between generations, and this happens so infrequently that it just isn’t an issue. On the other hand, testing your father would be worthwhile since you did not inherit his mtDNA. It is remotely possible that his mtDNA is the same as yours, but that would be pure coincidence!”

Unfortunately, Jasia was unable to test her father’s mtDNA directly, so she next wondered if there was a way to test someone else who might have her father’s mtDNA:

“Can I assume then that my brother’s DNA would give me the same information as my father’s would have? Would that be a correct assumption?”

My lengthy response explained that her brother could supply their father’s Y-DNA, but not his mtDNA:

“There are two main types of DNA testing – mtDNA and Y-DNA. mtDNA is found inside the mitochondria in all our cells, while Y-DNA is a chromosome that only males possess.

“mtDNA is passed from mother to child, both male and female. Notice, however, that only females pass on their mtDNA. Thus, you and your brother both have your mother’s mtDNA.

“Y-DNA, in comparison, is passed from father to son. Only males have Y-DNA. Thus, your brother has your father’s Y-DNA, but not his mtDNA.

“There are two great inheritance charts here and here that help explain the inheritance of mtDNA and Y-DNA.

“In response to your question, you can test your father’s Y-DNA by asking your brother, but that won’t work for mtDNA. To test your father’s mtDNA, you will need to find someone else who possesses that mtDNA. This could include your father’s mother, or your father’s brothers or sisters (if there are any), or your father’s sister’s children (i.e. your cousins). Note that you cannot test your father’s brother’s children, because they inherited their mtDNA from their mothers, who are not blood relatives to you.

“Absent any of these options, you just need to follow the mtDNA back to someone you can find. For instance, to test my grandfather’s mtDNA, I will have to work my way back through the family tree almost 150 years to find a branch that has a female line!!”

Note that I left out autosomal and X-DNA testing in my description. These are the two other areas of genetic genealogy testing, but they are either so new or so controversial that I decided to leave them out of my description. To learn a little bit more about these two additional types, see the “About Genetic Genealogy” page listed in my header.

Tomorrow we’ll have Part II of Genetic Genealogy Advice for Newbies!