It was 1953 when James Watson and Francis Crick solved the mystery of
identifying the molecular structure of the substance that was to be named
deoxyribonucleic acid - DNA. Once the "double helix" explanation
of how the strands of DNA actually fit together was understood, it provided
a means for microbiologists to recognize their patterns, analyze them, and
relate specific parts of DNA to biological phenomena and diseases. For this
discovery, Watson and Crick were awarded the Noble Prize in Medicine and
Physiology in 1962.
There are two types of DNA that are used in genetic genealogy and family
history research:
1). DNA from the Y chromosome that is called Y-DNA and it is present only
in males and transmitted from father to sons;
2). A small piece of DNA that occurs in the mitochondrion, outside of
the cell nucleus, is called mitochondrial or mtDNA. Males and females carry
this mtDNA but it is only transmitted through females.
To understand how this DNA is used to create genetic profiles, we need to look first at how DNA is a part of our chromosomes. The DNA is the genetic material that occurs in the nucleus of all of the cells in our body. In our cells the DNA is composed of two strands, twisted around themselves, to form the double helix. For all humans, the DNA molecules are arranged into 46 chromosomes which occur in 23 pairs. In 22 of these pairs the material is identical with one being inherited from the mother and the other being inherited from the father. However, the 23rd pair, which determines gender, is composed of either two X chromosomes - which produces a female - our an X and a Y chromosome - which produces a male. This matching of X and Y chromosomes that occurs at conception is possible because, while the female's egg always contains the X chromosome, the male's sperm may contain either an X or a Y chromosome.
This explains the often asked question of why the male Y-DNA is used
in genetic genealogy and why these patterns cannot be tracked using female
DNA. The Y chromosome and DNA are inherited only by a male from his father
and therefore it offers a unique record of paternal transmission. With the
female's two X chromosomes there is a recombination or "exchange"
of the DNA with that of the male's. This combining of DNA provides no opportunity
for unaltered transmission of DNA material, as with that of the Y-DNA.
.
Each male's unique Y-DNA are often referred to as "patterns,"
"signatures," or "fingerprints." The numbers ("alleles")
that you see on the Project Chart next to each of our volunteers' names
represent that individual's unique DNA profile based on 12 markers. Each
profile will be identical to that of their brother's, son's, father's, grandfather's,
great grandfather's.
The only exception to the continuation of this unique string of DNA markers is when a mutation occurs. The Y chromosome is composed of 60 million identifiable pieces, called "letters," that record each individual's unique history and make-up. These letters actually represent the combinations of four chemicals in the double helix that make up all DNA (Adenine, Thymine, Cytosine, and Guanine). As each individual's DNA is copied and passed from one generation to the next certain "copying errors" or mistakes may occur. This is like a "typo." Many of these are repaired through chemical means quickly and most have no effect on the life of the individual within whom the mistake occurred. However, since the Y chromosome is the only one of the 23 pairs that does not exchange DNA, the errors that occur here will represent changes that are passed on to the next generation.
These changes may represent the actual transposing of a letter, like an A for a T, or the insertion or deletion of letters or groups of letters. It has been estimated that a mistake or change in the basic DNA sequences may only take place once every 500 generations. If you look at our Project Chart you will see that the first DNA marker is called DYS 393. The numbers (alleles) for this marker from our volunteers range from 13 to 16. These represent the lengths or "repeats" that occur on this part of the chromosome. When you combine each of these markers, whether you use 12 or 25 markers, they will represent a unique pattern or profile. These profiles are also called haplotypes and may be compared to patterns that existed in early human development among unique ethnic and certain geographic human populations.
For the purposes of genetic genealogy, the closer that the numbers in
the profiles match, the more recent is the likelihood that those "matching"
individuals share a common ancestor. (Look at Robert's and Peter's 12/12
match in Cluster 1 and Vernon's and Craig's 11/12 match in Cluster 5.) Laboratories
identify either 12, 25 or 37 markers for genealogical research. Most surname
studies, such as ours, begin with the 12-markers which can be refined to
25 or 37 from the original sample at a later time. The 12 markers are sufficient
to differentiate basic family lines.
The interpretation of the differences of these markers is more complicated,
but it is important to understand so that proper conclusions can be drawn.
If there is a 12/12 match, as in Robert and Peter from England, then there
is a 99% probability that they are directly related. Where there is an 11/12
match (as with Vernon and Craig in Cluster 5) or 10/12 match (as with Kenneth
and Earnest Dale in Cluster 3) they will be "considered related,"
but the time frame at which their "most recent common ancestor (MRCA)"
occurred may have been more distant than for those with a 12/12 match. Profiles
with less than a 10/12 match, for analysis purposes, are usually not considered
to be related, at least within the time frame of recorded history. The actual
probability of defining relationships are as follows:
>A 12/12 match means that one's MRCA occurred between 1 and 62 generations
ago. There is a 50% probability that the MRCA lived 14 generations ago (350
years ago), or less, and a 95% probability that the MRCA lived within 62
generations ago (1550 years ago). Note: Scientists use 25 years to define
a generation for purposes of these predictions.
>An 11/12 match means that one's MRCA occurred between 1 and 122 generations
ago. There is a 50% probability that the MRCA lived 37 generations ago (925
years ago), or less; and a 95% probability that the MRCA lived 122 generations
ago (3050 years ago).
>A 10/12 match means that one's MRCA occurred between 1 and 166 generations
ago. There is a 50% probability that the MRCA lived 61 generations ago (1525
years ago), or less; and a 95% probability that the MRCA lived 166 generations
ago (4150 years ago).
These probability figures are supplied by the FamilyTreeDNA program,
with which our Project works. They also offer a DNA tutorial on their website:
www.familytreedna.com/facts.genes.asp if you would like more information.
Many female researchers have felt somewhat left out of this research process because of its reliance upon the male chromosome. There is a piece of DNA that occurs outside of the nucleus of our cells and within a membrane in the surrounding cytoplasm. Remember, the chromosomes are found within each cell's nucleus. This DNA is found within structures called mitochondria (mt) and the DNA itself is identified as mtDNA. Technically, this mitochondria is used by cells to convert oxygen into energy. However, buried within each piece of mitochondria is a small piece of DNA. Unlike the DNA in the nuclei of our cells that we inherit from both of our parents (the X and Y chomosomes), this mtDNA comes only from our mothers. While both males and females have mtDNA, it can only be passed on to offspring through the females. So while all of us carry this piece of mtDNA, we received it only from our mothers and it is only the females who can transmit it.
The Y-DNA is used in our surname research projects, in part, because it does not blend with the X-DNA, and precisely because it changes, or mutates, very slowly - estimated at one mutation about every 500 to 750 years. The mutations, or changes in an individual's DNA are the key to reconstructing genetic history. They allow for the tracking of the DNA profiles in male family lines over time. The mtDNA is unique in that it mutates even more slowly and that while all humans carry the mtDNA it is only passed on by females. These changes that are recorded in the mtDNA's "molecular clock" allow the researcher to identify and track broad patterns that have occurred over hundreds of thousands of years of human evolution.1
This mtDNA has also become an important resource in medical research.
If a woman carries a certain mutation in her mitochondrial DNA, that mutation
will be transmitted to all of her offspring. While her sons may inherit
this mutation they cannot pass it on. Researchers report that this type
of mutation, whatever the specific traits, are easy to recognize because
all of the children of an affected mother will also be affected and none
of the children of an affected father will be affected. Research has already
implicated certain mutations in the mtDNA with aspects of Parkinson and
Alzheimer diseases, and a form of epilepsy. 2
In this diagram above, females are represented by circles and males by squares.
All individuals shown in blue should have the same mitochondrial DNA sequence
because they are from the same maternal lineage. Thus, all three children
of the first-generation parents have the same mtDNA, but only the children
of the second-generation females share the same mtDNA sequences.
Brian Sykes, a professor of genetics at Oxford University in England, has become a well-known authority on DNA and its extraction from ancient sources. He and his team were the first in the world to recover traces of DNA in ancient human bones. In 1991 two hikers in the Italian Alps discovered the frozen remains of a male - later to be referred to as the "Iceman." Sykes successfully extracted mtDNA from the frozen bone fragments. The remains had been dated to between 5,000 and 5,350 years old by carbon dating methods. In his lab, Sykes compared the DNA profile of the "Iceman" with those in his database collected from a variety of living European samples. He found that the "Iceman" was definitely European. However, to his surprise, the "Iceman's" profile matched perfectly with a DNA profile collected from a lab worker - Marie, from Ireland. He observed: "This could only mean one thing. Marie was a relative of the Iceman himself. …there had to be an unbroken genetic link between Marie and the Iceman's mother, stretching back over five thousand years and faithfully recorded in the DNA.3
Sykes continued: "My research over the intervening decade has shown that almost everyone living in Europe can trace an unbroken genetic link (as Marie and the Iceman), way back into the remote past, to one of only seven women. These seven women are the direct maternal ancestors of virtually all 650 million modern Europeans. …I know that I am a descendant of Tara (one of the names given to the seven women, the "Seven daughters of Eve"), …I reckoned that Tara lived in northern Italy about 17,000 years ago. Europe was in the grip of the Ice Age, (but)….As the Ice Age loosened its grip, Tara's children moved round the coast into France. …Eventually Tara's children walked across the dry land that was to become the English Channel and moved right across to Ireland, from whose ancient Celtic kingdom the clan to Tara takes it name.4
Additional information about both the Y-DNA and the mtDNA tests and their
cost can be found on the Family
Tree DNA website.
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1 Brian Sykes (2001). The Seven Daughters of Eve. New York/London: W.W.
Norton, p.55
2 R. Scott Hawley & Catherine A. Mori (1999). The Human Genome: A User's
Guide. New York:
Academic Press, pp.79-80
3 Sykes, p.7
4 Sykes, pp.8-9