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,
Science
Send your questions to
dearscience@thestranger.com
