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DNA Testing: Choosing a test

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Genetic genealogy is a hot topic, but many people are put off by the learning curve. This is a place to get answers to your questions.

This project is sponsored by Geni's curators to provide help in choosing a DNA test and understanding what questions the different types of tests might answer.

How to participate

If you want to see the discussions from this project in your Message Center, click Follow.

If you have a question, start a discussion (or add on to an existing discussion). The curators and users who are interested in DNA testing will try to get you an answer.

To start a discussion, click the Project Discussion link in the upper right corner. Geni will take you to the Discussions page. Click Start a Discussion.

If you're a whizz at genetic genealogy and you want to help, consider becoming a Collaborator.

Understanding the test types

If you understand a few simple concepts, you will be able to understand what each type of test might prove.

Types of tests

For genealogical purposes there are three kinds of tests:

  • Autosomal DNA, for relatives on both sides within (about) the past four to seven generations - tests the autosomal DNA, which is inherited by children from both parents.
  • mtDNA, for the female line - tests the mitochondrial DNA in the cells, which is inherited by children from their mothers.
  • yDNA, for the male line - tests the y-chromosome, which is inherited by men from their fathers.

Autosomal DNA

Autosomal DNA is the combined DNA in 46 chromosomes each person inherits, 23 chromosomes from each parent. (Autosomal DNA also called nuclear DNA or recombinant DNA.) Recombination is random.

Autosomal test results can be used:

  • To find distant cousins related within about four to seven generations. Knowing there is a relationship, you can compare genealogical information in order to find clues, or prove suspected connections.

mtDNA tests

Unlike autosomal DNA, mitochondrial DNA (mtDNA) does not recombine. It passes intact to a woman's children without any contribution from the father. (Both men and woman have mtDNA.) The mutation rate for mtDNA is very low; perhaps one mutation at a given spot every 10-12 thousand years. Therefore, a family's mtDNA changes very slowly over many generations. Every person whose mtDNA contains a particular mutation shares a common maternal ancestry extending back thousands of years with every other person who has the same mutation.

mtDNA test results can be used:

  • To verify direct female lines through testing two or more descendants from a common direct female ancestor (NB: Full Sequence test FMS normally needed.)
  • To understand the remote origins of the human race.
  • To trace population movements in the time before recorded history.

yDNA tests

Unlike autosomal DNA, yDNA does not recombine. It passes intact from a man to his sons. (Women do not have y chromosomes, so they do not have yDNA.) The mutation rate for yDNA is very low, perhaps one mutation at a given spot every 14 thousand years. Therefore, a family’s yDNA changes very slowly over many generations. Every man whose y chromosome contains a particular mutation shares a common paternal ancestry extending back thousands of years with every other man who has the same mutation.

yDNA test results can be used:

  • To verify direct male lines through testing two or more descendants from a common direct male ancestor
  • To understand the remote origins of the human race.
  • To trace population movements in the time before recorded history.

For cultures where hereditary patrilineal surnames have been in use for hundreds of years

  • To show that two men with the same surname have a common ancestor, who must have lived after the time when surnames were adopted.
  • To show that two men with different surnames have a common ancestor, who might have lived before the time when surnames were adopted.
  • To show that men with the same surname do not belong to the same male line, and therefore do not have a common ancestor.

Examples

Example 1: Simple yDNA test

Family tradition says that colonial immigrants John Smith and William Smith were brothers, but there is no paper evidence.

This is a classic case where DNA tests on male-line descendants of the two men can support or disprove the tradition. If a male line descendant of John Smith matches a male line descendant of William Smith, the two men probably belong to the same male line. They might (or might not) have been brothers, but they were certainly relatives.

If they do not match, family tradition might be wrong. To be fully certain, you should find additional male-line descendants to be tested. This will ensure that the DNA of the living men really reflects the DNA of their apparent ancestor.

A yDNA test at the 12-marker level will be enough to show whether the two men match. If they match at the 37-marker level there is a 95 percent probability that their common ancestor lived within the past 8 generations. The more markers tested, the higher the certainty.

Example 2: Simple mtDNA test

Ancestor John Smith had two wives, Mary Williams and Alice Jones. The older children were Mary's, but it's not clear whether the younger children belonged to Mary or Alice.

This problem might be partially answered by DNA. A test on a female-line descendant of Mary Williams will establish the pattern for her mtDNA. Then, that pattern can be compared to a female-line descendant of one of the younger children. If they match, the two children belonged to the same female line, so they probably had the same mother. If they do not match, they had different mothers.

If any of the younger children were boys, their children were not female-line descendants of the boy's mother so this strategy will not work. The strategy also will not work if the younger children who were girls do not have living female line descendants.

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