Determining the biological relationship between two people through DNA testing is simple, inexpensive, and more accurate than ever before. Paternity testing is the most common form of DNA testing, but relationships such as siblings, grandparents, aunts, or uncles can also be proven or disproven. Tests are either performed by professionals at a clinic, or by yourself using a home kit. In fact, DNA testing can even be done postmortem. And now, prenatal paternity tests take the stress out of pregnancy with no risk to the unborn. While a cheek swab is the most common method of obtaining DNA for a test, alternatives like hair, semen, or fingernails can be used.
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About DNA Testing
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Which Relationships can be Proven>
The following relationships can be established via DNA testing:
- - Father to child
- - Mother to child
- - Grandparent to grandchild
- - Aunt or Uncle to niece or nephew
- - Brother to Sister (Sibling to Sibling)
- - Half-brother to Half-sister (Half-sibling to half-sibling)
- - Identical Twins vs. Fraternal Twins
How does DNA Testing Work?>
DNA testing to prove - or rule out - close family relationships is a three step process.
- 1) at home or in a clinic: collect specimens of 2 or more parties
- 2) at a laboratory: extract the DNA from the specimens
- 3) in a PhD's office: compare the allele sizes of the 21 chromosomes and run a statistical analysis to determine the likelihood of the relationship between the parties.
What Specimens Contain DNA?>
DNA can be extracted from a variety of specimens. The easiest to obtain, and therefore the most common source of genetic material for relationship testing, is buccal cells. Buccal cells are the skin cells of the inner cheek, which rub off easily onto cotton swabs.
When a technician rubs a cotton swab inside the mouth of a person getting a DNA test, they are not testing the saliva. Rather, the invisible cells that get latched onto the swab contain DNA which can be extracted in a lab. Other specimens, like hair follicles or fingernails, also contain DNA. However, it is more difficult for the lab to extract DNA from non-cheek swab specimens.
How Accurate is DNA Testing?>
The results of a DNA test are given as a probability, which displays the statistical odds that two people are related in the way in which they think they might be. A 21 marker paternity or maternity test will always come back with a result of either 0% or greater than 99.9%.
Other relationship tests, e.g., sibling or avuncular studies, may have a broader range of possible results, and can even be inconclusive. Adding additional close relatives to an inconclusive test can reduce the uncertainty and help the lab get closer to a conclusive result.
Why do DNA test results vary?>
Each of us gets half his DNA from his mom, and half from his dad. There are 2 alleles on each chromosome, one from each parent. So, if you look at a child and her parent, one of the alleles in each chromosome will always match (unless, in rare cases, there was a mutation).
If you have a full sibling, then people say you are related by "half". However, that's an average. Your DNA may have as much as 75% in common, or as little as 25%. You both get one allele from mom and one from dad at each marker. However, mom and dad both have 2 alleles at each marker, and the one they gave you is not necessarily the one they gave your sibling.
If you have a half-sibling, then it can be said you are related by a "quarter". Again, that's an average. You may share more than a quarter of your DNA, or less. The alleles that your shared parent gave you may or may not be the same.
As you can see, as you get further away from the primary maternal and paternal relationships, it gets harder and harder to determine with certainty if two people are related by blood. The whims of nature in determining which child inherits which allele creates uncertainty in the analysis, resulting in certain (more distant) relationship DNA tests occasionally coming back inconclusive.