Dear Science,

I had a DNA test done in Seattle, the alleged daughter had hers done in Michigan, and then they shipped them both off for lab work. It came back that I'm not the father, but she looks a lot like me. Do you think it's worth another try? Is a blood test as good as a DNA test?

Your Daddy?

Paternity testing is all about mixing and matching.

DNA-based tests work by comparing the recipes—the alleles—you have for a given gene to those of a possible child. For most genes, we get two alleles—one from our mother and one from our father. A child of yours must have one of your alleles for all the paired genes in his or her DNA.

The blood-type gene is located on chromosome 9; you can think of this like an address. In the human population, there are three major alleles running around—A, B, and O. These are the possible versions of this gene that can live at the address. Since all of us have two copies of chromosome 9, we each get two alleles for this gene. As an example, if the mother is blood type B, the child is blood type A, and you're blood type O, you cannot be the father. The child's blood type A allele had to come from somewhere, and neither you nor the mother have a copy. Some other man, with a type A allele to give could be the father. Matching on a single gene isn't good enough to really say you are the father; mismatching once is good enough to say you probably aren't. So, ruling out paternity is much easier than definitely establishing you are the dad. Most DNA-based paternity tests look at a whole bunch of genes, trying to match the child's alleles to those of the possible parents. The more genes looked at, where all match, the better the chance you are actually the father.

The false negative rate of the test—how often it will say you're not the father, when you actually are—is very tiny. For that to happen, both copies of the gene in the child would not match any of the copies you have. A major mutation would have to occur for that result, and that's highly unlikely. It is possible the samples were switched, and therefore even if the test worked properly, the results could still be wrong. Only for that reason would I retest.

Even if you are not the biological father, you can still be a parent. Study after study have shown that nonbiological parents can do as good of a job raising a child as biological parents. If you love her, she can be your daughter, regardless of what any DNA test says.

Parentally Yours,


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