Autosomal DNA is normally diluted as it gets passed to the next generation, since DNA from the mother competes with the DNA from the father for the opportunity to pass on genetic material. But some genetic material tends to "clump" together and gets passed down to successive generations as blocks.
The autosomal DNA test looks for long blocks. Family Finder considers blocks of 10 centiMorgans or longer as indicative of "conclusive shared ancestry."
Eventually, such a block of genetic material runs out of luck and gets split apart or doesn't get passed on. Therefore, a good test strategy is to have the oldest available generation in the line you are researching take the test. A parent, aunt, uncle, or grandparent would be an excellent test subject.
This dilution does not occur with mtDNA or Y-DNA tests.
The six generation pedigree example below shows that if two 4th cousins test there is about a 50% probability they will match and for fifth cousins the chances are only 10%.
- Ancestors, Year of Birth, Relatives, Probability of Match
- 1 you, 1960, 100%
- 2 parents, 1930, siblings, 100%
- 4 grand parents, 1900, 1st cousins, 100%
- 8 g-grand parents, 1870, 2nd cousins, >99%
- 16 g-g grand parents, 1840, 3rd cousins, >90%
- 32 g-g-g grand parents, 1810, 4th cousins, >50%
- 64 g-g-g-g grand parents, 1780, 5th cousins, >10%
Thus, a practical limit to this test would be the detection of fourth cousins sharing common sets of gg-grandparents with you. However, it can be the case that two more closely related people could share surprisingly little DNA and conversely, two more distantly related people share more DNA than might be expected. It depends on what you "win" in the gene lottery and what has been passed down to you. If you can afford to do so, testing siblings and cousins can greatly expand your domain of matches and give you more possibilities to sift through.
Because autosomal genes get recombined and thus diluted with each successive generation, then ideally, when your funds are limited, it is better to test the oldest available generation in your family who is descended from the line you are researching. If you have a parent, aunt, uncle, or grandparent who is descended from your Driscoll ancestor of interest, devote your limited funds to having that person tested.
Once your FF test results are completed and you've joined the DNA project, consider submitting information on your Driscoll ancestral line, such as census records, land valuation records, birth, marriage and death dates and locations, the names and birthdates of children, etc. Label your information with your test kit number. View some of the existing family history data for examples.
Census Records | Vital Records | Family Trees & Communities | Immigration Records | Military Records Directories & Member Lists | Family & Local Histories | Newspapers & Periodicals | Court, Land & Probate | Finding Aids