17 June 2010

Three DNA Tests for Genealogy, part 2 Y-DNA & mtDNA

Y-DNA Testing

The Y-chromosome has been passed down from father to son virtually unchanged since mankind began. The small changes in that Y-chromosome help us separate people into family units. This test gives results for the all male line, the top line of a pedigree chart, when the male tester is number one on that chart. See SUCCESS STORIES at www.isogg.org for Y-DNA testing used to break through a dead-end paper trail.

For genealogy, particular markers are tested on the Y-chromosome as they provide a mix of slowly and quickly mutating samples. This helps find which men are more closely related than others. If all the markers used were slow to mutate more people would appear to be closely related. If most markers were quick to mutate most people who are related would appear not to be. The geneticists and mathematicians collaborate to get the correct mix.

Each marker is tested for short tandem repeats (STRs). Our DNA, in part, is made up of four chemical bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C). These chemical bases form short patterns (AGAT) which appear side by side (in tandem), hence Short Tandem Repeat. The STRs are counted, and this count is the result given for a particular marker. For example, if the STR marker DYS393 repeated its AGAT sequence fourteen times, the result for that marker would be a 14. Different markers have different chemical patterns, but in each tested marker you receive your results as the number of times the pattern is repeated.

The result of testing is a series of numbers called a haplotype. (DYS393 is 14; DYS390 is 22; DYS19 is 15, etc.) This is your DNA signature. However, it is not just your signature, but it is that of all the males on your all male line. Of course, there could be a mutation at any time in that male lineage which would change how many times the repeat is seen, but you are still closely related.

A test for 12, 25, 37, or 67 STR markers can be ordered. The more markers you match with others the closer the common ancestor is. For example, if you take three traits you have (eye color, shape of your ears, skin tone) and compare only those three with your friends and neighbors, you are likely to match several people. If you now add ten more traits, you narrow that pool of matches. It is the same idea between testing twelve markers as opposed to thirty-seven or more.

For a close match you can have the exact same markers as another tester or a few differences. The number of differences allowed to still remain a good match is determined by the number of markers you test and the testing company’s prediction to the closest common ancestor. These predictions for the closest common ancestor are based on mathematical probability.

Y-DNA summary: · Only males can test their Y-DNA.
· The test gives matches for the all male line.
· Short Tandem Repeats (STRs) are recorded as the result of testing.
· The more STR marker results you share with someone the closer your common ancestor.

mtDNA Testing

The mitochondria is outside the cell’s nucleus. It is not the sex chromosome like the Y is. This genome is inherited by men and women from their mothers. Mothers have passed their mtDNA from mother to all their children virtually unchanged since womankind began. The mitochondria is very slow in mutating so it is more useful for ancient ancestry or to help with a specific problem where good testing candidates are available. This test gives result for the all female line, the bottom line of a pedigree chart, when the male or female tester is number one on that chart. See SUCCESS STORIES at www.isogg.org for several ways mtDNA has been used to break through a brick wall.

There are three mtDNA tests at Family Tree DNA: HVR1 (Hyperveritable region 1), HVR2 Hyperveritable region 2) and the FGS (Full Genomic Sequence). The first two are parts of the mitochondria while the last is the entire mitochondria.

This test looks at Single Nucleotide Polymorphisms (SNPs). A SNP (pronounced SNiP) is a change in a single letter of our genetic code (A, G, T, C). For mtDNA tests, results are compared against the Cambridge Reference System (CRS). You are given only the differences your results from the CRS as the FGS contains 16,568 markers. No one wants to put all that on a website or frame-able certificate! Examples of mtDNA marker results are: 16256T; 16399G; 315.1C. The numbers in this case are the names of the markers. The letter after the number is the chemical base (Thymine, Guanine, Cytosine) that differs from the CRS.

For a match, you need to have the exact markers as someone else. Even then, the time to the common ancestor could be before surnames and before recorded genealogical data.

mtDNA summary:
· Males and females can test their mtDNA.
· The test gives matches for the all female line.
· Single Nucleotide Polymorphisms (SNPs) are tested and compared against the CRS (Cambridge Reference System).
· The more markers you share with someone the closer your common ancestor, but given the slowness in mutating that match could still be hundreds to thousands of years ago.

Next, Autosomal Testing, a new breakthrough in genetic genealogy!

copyright 17 Jun 2010, E. Aulicino
Thank you R.

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