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We at Upfront with NGS are
very excited to bring you this four part series on DNA testing and its
genealogical uses ...
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 Va. , Tn.
and Ky.
Check back tomorrow
for Part 2!
Roberta Estes
www.dna-explained.com
(blog)
Copyright
2004-2013, DNAeXplain, all rights reserved.
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