05 August 2013
We at Upfront with NGS are very excited to bring you this four part series on DNA testing and its genealogical uses ...
Article courtesy of Roberta Estes, www.dnaexplain.com,
Graphics courtesy of Roberta, Family Tree DNA, www.familytreedna.com and the ISOGG wiki at http://www.isogg.org/wiki/Wiki_Welcome_Page.
DNA testing for genealogy didn’t exist a few years ago. In 1999, the first tests were performed for genetic genealogy and this wonderful tool which would revolutionize genealogy forever was born into the consumer marketplace from the halls of academia, thanks to one very persistent genealogist, Bennett Greenspan, now President of Family Tree DNA.
Initially we had more questions than answers. If it’s true that we have some amount of DNA from all of our ancestors, how can we tell which pieces are from which ancestor? How much can we learn from our DNA? Where did we come from both individually and as population subgroups? How can DNA help me knock down those genealogy brick walls?
In just a few short years, we have answers for most of these questions. However, in this still infant science we continue to learn every day. But before we discuss the answers, let’s talk for just a minute about how DNA works.
DNA - The Basics
Every human has 23 pairs of chromosomes (think of them as recipe books), which contain most of your DNA, functional units of which are known as genes (think of them as chapters). One chromosome of each pair comes from a person’s mother and the other from their father. Due to the mixing, called recombination, of DNA that occurs during meiosis prior to sperm and egg development, each chromosome in 22 of the 23 pairs, which are known as autosomes, has DNA (think of it as ingredients) from both the corresponding parent’s parents (and their ancestors before them).
Two portions of our DNA are not combined with that of the other parent. The 23rd chromosome, in the box above, determines the sex of the individual. Two X chromosomes produce a female and an X and a Y chromosome produce a male. Women do not have a Y chromosome (otherwise they would be males) so they cannot contribute a Y chromosome to male offspring. Given this scenario, males inherit their father’s Y chromosome unmixed with the mother’s DNA, and an X chromosome from their mother, unmixed with their father’s DNA.
This inheritance pattern is what makes it possible for us to use the Y chromosome to compare against other men of the same surname to see if they share a common ancestor, because if they do, their Y chromosome DNA will match, either exactly or nearly so, because it has been passed intact directly from those paternal ancestors.
Autosomal DNA, X chromosomal DNA and, in males, Y chromosomal DNA are all found in the nucleus of a cell. A fourth type of DNA call mitochondrial DNA, or mtDNA for short, resides within cells but outside the cell’s nucleus. Mitochondrial DNA packets are the cell’s powerhouse as they provide the entire body with energy.
For both genders, mitochondria DNA is inherited only from the mother. Men inherit their mother’s mtDNA, but do not pass it on to their offspring. Women have their mother’s mtDNA and pass it to both their female and male offspring. Given this scenario, women inherit their mother’s mtDNA unmixed with the father’s and pass it on generation to generation from female to female. This inheritance pattern is what makes it possible for us to compare our mtDNA with that of others to determine whether we share a common maternal ancestor.
Autosomal DNA, the rest of your DNA, those other 22 chromosomes that are not the X/Y chromosome and not the mitochondrial DNA, tends to be transferred in groupings, which ultimately give us traits like Mother’s blue eyes, Grandpa’s chin or Dad’s stocky build. Sometimes these inherited traits can be less positive, like deformities, diseases or tendencies like alcoholism. How this occurs and what genes or combinations of genes are responsible for transferring particular traits is still being deciphered.
Sometimes we inherit conflicting genes from our parents and the resolution of which trait is exhibited is called gene expression. For example, if you inherit a gene for blue eyes and brown eyes, you can’t have both, so the complex process of gene expression determines which color of eyes you will have. However, this type of genetics along with medical genetics does not concern us when we are using genetics for genealogy. Let’s focus initially on the unrecombined Y chromosomal DNA, called Y-line for short, and mtDNA as genealogical tools.
How Can Unrecombined DNA Help Us With Genealogy?
I’m so glad you asked.
During normal cell combination, called meiosis, each ancestor’s autosomal DNA gets watered down or divided by roughly half with each generation, meaning each child gets half of the DNA carried by each parent.
However, that isn’t true of the Y-line or mtDNA. In the following example of just 4 generations, we see that the Y chromosome, the blue box on the left, is passed down the paternal line intact and the son has the exact same Y-line DNA as his paternal great-grandfather.
Similarly, the round red doughnut shaped O represents the mitochondrial DNA (mtDNA) and it is passed down the maternal side, so both the daughter and the son will have the exact same mtDNA as the maternal great-grandmother (but only the female child will pass it on).
The good news is that you may well have noticed that the surname is passed down the same blue paternal path, so if this is a Jones family, the Y-line DNA travels right along with the surname. How it can help us with genealogy now becomes obvious, because if we can test different male descendents who also bear the Jones surname, if they share a common ancestor somewhere in recent time (the last several hundred years), their DNA will match, or nearly so. Surname projects have been created by volunteer administrators at Family Tree DNA to facilitate coordination and comparison of individuals carrying the same or similar surnames.
Mitochondrial DNA (mtDNA) is useful as well, but not as easily for genealogical purposes since the maternal surname traditionally changes with each generation.
There have been several remarkable success stories using mtDNA, but they are typically more difficult to coordinate because of the challenges presented by the last name changes. Sometimes joining regional projects is more useful for finding mtDNA matches than joining surname projects. A case in point is the Cumberland Gap projects, both Y-line and mtDNA, which have helped many people whose families lived in close proximity of the Cumberland Gap (at the intersection of
connect with their genetic cousins. What
mtDNA as well as Y-line DNA testing can easily do for us is to confirm, or put
to bed forever, rumors of Native American, European, African or Asian ancestry
in that direct line. Ky.
Check back tomorrow for Part 2!
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