If a mother had a child with her own son, could a paternity test yield positive result for the son's father?

Question

Let's imagine a family composed of Mr A and Mrs B, who have a son named John.

Now, if for whatever twisted reason, John had a child with Mrs B, could a paternity test yield positive result for Mr A ?

Since John's genes all come from Mr A and Mrs B, the child's genes would all come originally from Mr A and Mrs B as well. Thus it seems to me like the test could suggest Mr A is the father of the child.

Or would there definetly be some "red flags" that suggest something wrong happened ? Like, for example, if the child is a girl, that would mean both of her X chromosomes originally came from Mrs A (since John's X chromosome comes from his mother, as for all males), possibly even the same chromosome twice.

Let's consider that only Mr. A is tested, and not John. Would the child be considered genetically close enough to Mr. A that a test could attest that he is probably the child's father, granting there is no other test showing someone else as a much more likely candidate ?

Answer 1

Let's approach some different methods of distinguishing parents:

1: The Y chromosome. These are passed down from father to son. Therefore, if the baby is male, its Y chromosome will be the same as John's and his father's - it will be indistinguishable who it's from.

2: The X chromosome. A male baby always has one of the mother's X chromosomes. A female baby always has its father's X and one of the mother's X. A male baby of John and Mrs B would always end up with one of Mrs B's X chromosomes, meaning that this can't distinguish the father. A female baby would have two X chromosomes from Mrs B (because John's own X came from her), proving that it is the product of incest.

3: Mitochondrial DNA. mtDNA is passed on from mothers to their children, never from the father. Therefore regardless of whether John or Mr A is the father, the baby would have Mrs B's mtDNA - not useful.

4: DNA fingerprint. This is primarily based on stretches of the human genome that contain repeats of short DNA sequences (so-called STR/short tandem repeats, or VNTR/variable number tandem repeats). In both cases, the sequence is the same in individuals, but the number of repeats is extremely variable and each human therefore has a (possibly) unique combination across the different repeat locations in the genome. Because children inherit 50% of their DNA sequences from the father and 50% from the mother, John himself will have his mother's characteristic lengths in 50% of repeat locations and his father's in the remaining 50% of locations (roughly). A child of John and Mrs B would have MrA's repeat lengths in 25% of locations and Mrs B's in 75%. The fractions "100% Mr A" vs. "50% Mr A" vs. "25% Mr A" can be distinguished by this method.

Answer 2

Sorry to answer a really old question but I've been researching this exact topic and this post often appears near the top of search engine results, and I think it could use some more info; the top answer does a great job outlining the technical factors at play, but it neglects to consider a couple of practical factors that will affect the outcome of paternity testing in the real world. I am going to assume we are using commercial paternity testing services widely available to the general public, and not a service that is specifically more precise.

1. Because only Mr. A is being tested and not John, there is a significant possibility that commercial paternity testing services will return a "false positive" for Mr. A being the father. These testing services have articles specifically mentioning scenarios just like this one, sans mention of incest. According to these articles, it is not unthinkably uncommon for two potential fathers to be closely related, i.e. brothers or father and son. Quote: "When the two potential fathers share a father and son relationship, it must be mentioned to the testing facility beforehand, as failing to mention this might result in a high false-positive result." The ubiquity of this kind of disclaimer across articles from multiple different paternity testing services suggests that false positives in scenarios like this one are not only possible, but especially likely if the testing service is not made aware of any of the broader context.

2. What are the chances that Mrs. B participates in the test? This reduces the likelihood of a false-positive scenario. Quote: "[...] your best chance of receiving an accurate result lies in both parties agreeing to take the test. But in this case, the likelihood of success further increases if the mother agrees to undergo a test as well, which will help isolate the genetics of the offspring to an easier degree." So even if John does not participate at all, if Mrs. B participates, it will reduce the chances of a false-positive scenario. For most commercial paternity testing services, the participation of the mother is an optional feature offered at additional cost for greater certainty. Greater certainty would only be required if there was reason to doubt the results of a standard paternity test, so in a scenario that makes the fewest assumptions, Mrs. B does not participate, leaving increased probability of a false-positive.

3. This is probably the biggest factor overall — will the paternity testers be specifically told anything that might indicate the situation is non-standard? Or will it just be a blind test of "Is Mr. A likely the father of Mrs. B's latest child?" Even if the testing service is not advised of specific details like the possibility of incest, if any questions of the test's potential validity are raised, they are highly likely to ask for additional samples, if for no other reason than to upsell you — these are commercial services after all.

In summary, for a real world commercial paternity testing scenario, it appears to be a numbers game that's difficult to predict; if the test really is just a DNA sample of Mr. A and the child with no additional context, then yes, there is a strong possibility that a testing service will determine it's "close enough" and send back a "positive" result. They quite literally admit this. The likelihood of that outcome depends on the policies of the testing service and the personal standards of the personnel doing the testing (and, lets be honest, whether their lunch break is about to start!) whether a "close" result like this one would be considered "close enough" to send back positive, or whether it warranted a closer look and/or request for additional samples.

Sources:

1. Home DNA Direct
2. DNA Center
3. Easy-DNA

Answer 3

No, Mr. A would not be flagged the father. This is becasue when a parental test is done they look to what genes have the mother, the father and the baby. First they look at what genes has the mother and the baby. The one missing should be from the father.

In this case it could be possible that for example:

Mr. A: 1 gene Red hair, gene Blond hair.
Mrs. B: gene Brown hair, gene Black hair.
John: gene Brown hair (from his mother), gene Blond hair (from his father).

If the baby has: gene Black hair (from mother) and gene Brown hair (from John. Then that would prove the hair color is not from mr. A. He doesn't have those genes. So he isn't the father.