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Genetic genealogy

I have been exploring the use of DNA in family history research.

DNA

DNA is a complex organic molecule that stores enormous amounts of information which controls how our body grows, by providing code to make the different protein molecules which make up different parts of the body. DNA is made up of two strands which curl around each other in the famous “double helix” shape. Along each strand are nucleotides (a sugar based molecule), each of which links with a nucleotide on the other strand, so the structure is akin to a coiled ladder, with each rung made up of a nucleotide pair. There are four types of nucleotides, labelled A, C, G and T after their chemical names, and are sometimes called DNA bases. The human genome has more than two billion bases.
dnastructure

DNA molecules are grouped into 23 chromosome pairs – 22 autosome pairs, which control most of our body’s characteristics, and a sex chromosome pair (either XY for men or XX for women).  These are found in every cell in our body. However only a small portion of the DNA is used to make protein in cells; the rest is sometimes called junk DNA. In addition, information is stored in mitochondrial DNA, found in a different part of each cell, and passed on from mother to child.

Genes are segments of a chromosome that determine particular physical attributes. Humans have two copies of every gene, one from each parent and each one is different (i.e. with different base pairs) – these different forms of the same gene are called alleles. Humans have more than 20,000 genes in total. Most genes are the same in all people, but some are different, giving the wide variety in human bodies and personalities.

Reproduction

Cells reproduce by dividing and reproducing their DNA almost exactly. But recombination can occur, where segments of DNA are swapped between two chromosome pairs, thus creating a new DNA. Occasionally mutations also occur, though few of these are viable and passed on.

When a child is conceived, one DNA strand is copied from each parent so that a new DNA is created, half from each parent. But recombination when the sperm and egg cells are produced means that there are subtle, more or less random, but important differences in the DNA passed on to each child. Thus we all inherit half our DNA from each parent, but only approximately a quarter from each grandparent, and so on.

Genetic genealogy

Because we inherit our DNA from both parents, with subtle changes due to recombination, we will have many long segments of each chromosome the same as each parent, fewer long segments the same as each of our four grandparents, and reducing amounts in common as we go back up our family tree. Thus if two people have long segments of DNA in common, it is highly likely, though not certain, that they have a relatively recent common ancestor. The larger amount in common, the more recent the common ancestor is likely to be. The amount of DNA in common is measured in CentiMorgans (cM), which give an indication of the probability of a family match.

DNA testing is carried out by identifying the base pairs in long segments of DNA. Four basic tests are available:

  • Autosomal tests identify the bases at about 700,000 locations on the 22 autosomal chromosomes that are known to vary across humanity. Matches can identify relations up to about 5th or 6th cousins.
  • Y DNA is only carried by men, so this test can provide information of the male line – in effect the surname line – and can give an indication of deep ancestry (i.e. back many generations to a common ancestor).
  • Mitochondrial DNA is carried by both genders but is passed on only by mothers. It gives an indication of deep ancestry back to tens of thousands of years, by identifying a haplogroup, which identifies a common ancestor and location.
  • X DNA is found in the 23 chromosome of both genders. It can be tested separately, but is often included in autosomal testing.

DNA testing can reveal relationships that were previously unknown. If two people who have tested have several large segments in common, then they can conclude they have common ancestry, and begin to search documentary records to discover their common ancestry. Once two people have identified a common ancestor, they can be confident that anyone else who shares the same common segments has also descended from the same common ancestor, whereas someone with a match on a completely different set of segments probably comes from a different branch of the family.

DNA testing and matching

Autosomal tests cost about $US100, with other tests costing $200-$300. There are three main genealogical testing companies, 23 and Me, Ancestry and Family Tree DNA. All companies have their advantages and disadvantages. For example:

  • Only FTDNA does a full range of autosomal, mtDNA and Y-DNA testing. The others only do autosomal (23andMe includes a small amount of mtDNA and Y-DNA in its autosomal testing).
  • Ancestry’s comparison tools are not as comprehensive as the others two, but they can provide links to the large number of Ancestry family trees online (though it costs extra to gain access to these).
  • The Ancestry and 23andMe databases are larger than FTDNA’s (important because this provides a larger pool of possible matches) but FTDNA has more people from outside the USA.

It is possible to examine matches obtained by one of the other companies by (a) transferring Ancestry and some 23andMe results to the FTDNA database for a small fee, or (b) uploading results from any company to the free Gedmatch site, to increase the number of matches to investigate. If you are serious about using DNA results for genealogy, this is a no brainer.

McNaughton family history and DNA testing

I’m not aware of any DNA testing of anyone in this family tree. I have had an autosomal DNA test performed (by FTDNA), but I am not personally part of this family. It is possible some testing will be done soon, and I would be interested in hearing from anyone connected to this tree who has done a DNA test.

DNA diagram by US National Library of Medicine.

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