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 ?