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Genetic Genealogy Basics
By Herb McDaniel – 15 March, 2013 to Bainbridge Island Genealogical Society
Genetic genealogy is the use of DNA to explore ancestral origins and relationships between individuals. There are four types of genetic genealogy tests yDNA -‐ males only for paternal line mtDNA -‐ males and females for maternal line atDNA -‐ males and females for relationships xDNA -‐ not normally offered The results of a genetic genealogy test do not include a family tree. DNA alone cannot tell a person who their great-‐grandmother was, or what Italian village their great-‐great grandfather came from. Genetic genealogy is an addition to traditional genealogical research, not a replacement. The University of Utah has a great web site for learning about the types of DNA testing.
http://learn.genetics.utah.edu/content/extras/molgen/auto_dna.html
yDNA Testing yDNA tests, available only to males, examine either STRs (short tandem repeats) or SNPs (single nucleotide polymorphisms) on the Y chromosome. For an STR test, short segments of DNA are measured. The number of repeats in that short sequence changes over time, and these changes are passed on from father to son. STR analysis provides a person’s haplotype, which is used to predict an individual’s haplogroup. SNP tests examine single nucleotide changes in the DNA sequence and are typically used to determine a person’s exact haplogroup. To potentially find other people who match our haplotype, we use two resources: the yDNA database of the testing company and free public yDNA databases like ySearch. The testing company automatically checks customers’ results for matches within the company database. Single Nucleotide Polymorphism (SNP) testing examines single nucleotides at specific locations on the Y chromosome (compared to STR marker testing, which measures short regions of DNA). Advanced SNP testing or deep clade testing is available after STR testing. Since a mutation at a single base is very rare compared to changes in STRs, males who share a SNP usually share an ancestor who lived many generations or many hundreds of generations ago. For this reason, SNPs have been used to identify the branches in the Y-‐chromosome family tree and define Haplogroups, as well as very, very distantly related people.
mtDNA Testing Mitochondria have their own DNA and are passed down only from mothers to both male and female offspring mtDNA tests, available to both males and females, sequence a short region of the mitochondrial genome and a new full mitochondria sequence (FMS) test is now available. A region of the DNA is sequenced for mutations. The mtDNA sequence is then compared to a single mtDNA sequence, the Cambridge Reference Sequence. The differences are listed as mutations that can be compared to the thousands of other mtDNA mutation listings. The results of an mtDNA test provide a person’s haplotype. Companies also perform additional testing with every mtDNA test to determine an individual’s maternal haplogroup. The results can also be used to estimate the amount of time to which individuals share a most recent common ancestor (MRCA). To potentially find other people who match our haplotype, we will use two resources: the mtDNA database of the testing company and free public mtDNA databases like mitoSearch. The testing company automatically checks customers’ results for matches within the company database as long as the customer has signed the release form allowing their name and email to be shared with their matches. Your yDNA and mtDNA are like a laser beam -‐ they look straight up your all-‐male and all-‐female lines. They are very precise, but can only find cousins from your ancestors that are on that line. Go back, say 7 generations, where you have 128 ancestors. These tests only go to one of those ancestors. So in this case yDNA and mtDNA results will help you find 1/128th, or less than one percent, of your cousins. This is a powerful tool but only covers a very small percentage of your ancestry. And the cousins it finds must also be on an all-‐male or all-‐female line -‐ so this also cuts down on the number of cousins you can find with this test.
atDNA Testing
This autosomal DNA (atDNA) test, available for both men and women, examines SNPs (single nucleotide polymorphisms) across the 22 autosomal chromosome pairs. These chromosomes contain DNA from all sides of your family.
The atDNA test examines hundreds of thousands of SNP positions and uses the results to find recent relatives and geographic origins. Autosomal chromosomes, or autosomal DNA, are inherited from both parents and mixed from one generation to the next.
Every person has 46 chromosomes in total, 23 from each parent. 2 of these 46 chromosomes are the sex chromosomes. The other 44 are autosomal chromosomes.
The atDNA test, available for both men and women, examines SNPs (single nucleotide polymorphisms) across the 22 autosomal chromosome pairs. These chromosomes contain DNA from all sides of your family.
Each pair of autosomes are randomly jumbled as they are passed through each generation in a process called recombination and include DNA from all of your ancestors This atDNA test can find almost any of your 2nd or 3rd cousins who test, but falls off with each generation further out, and reports fewer and fewer persons who may actually be cousins. FTDNA has posted the probabilities of a specific relative passing their filter and showing up as a match with us. Parents and 1st cousins -‐ virtually 100 percent 2nd cousins about 99 percent 3rd cousins about 90 percent 4th cousins about 50 percent and declining percentages beyond that As opposed to the laser-‐beams of mt or yDNA tests, autosomal DNA testing is more of a shotgun. Your matches are from all over your ancestry. After you get the names and emails of your new "cousins", you're back to old fashioned genealogy. You need to share pedigree or GEDCOM files with each of them to find your common ancestors. A. Some won't answer your emails – privacy concerns or no interest in geneealogy B. Some are adopted, or otherwise know little of their ancestry C. Some may have a common ancestor with you that is not documented by both of you, so you can't readily find the match The results of any genetic genealogy DNA test do not include a family tree. DNA alone cannot tell a person who their great-‐grandmother was, or what Irish county their great-‐great-‐grandfather came from. You must have a pedigree chart or GEDCOM file for comparison with those of your matches.
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