Alternate paternity is often invoked to explain incongruities in the results of familial genetic testing. It is estimated that the frequency of alternate paternity is about 5% (i.e., every 20 patients randomly tested for any disease where one cannot be recognized to be the real son of the legal father), although many colleagues insist on saying the number is even higher. The scientific community has largely discussed this issue and has defined specific guidelines on how to address the topic during the genetic counselling.
First of all, it is very important to remember that the primary intent of a clinical genetic testing is finding the disease-causing mutation and nothing else. Hence, any possible issue that arises as secondary to this first diagnostic purpose must be attentively evaluated before discussing it with the patients and their family. For example, a couple sending their child’s DNA to prove the diagnosis in the son may have absolutely no interest in being puzzled by possible doubts on paternity. Therefore, clearly addressing such an issue in case of incongruities arising from family testing (i.e., in case some genetic variants of the child cannot be detected in the father) might have a big emotional impact and lead to severe disruptions within the familial relationships.
However, inconsistent results from family testing are a recurrent problem in genetic counselling and facing them is sometimes necessary to conclude the diagnostic process, requiring further genetic investigations and making the discussion with the patient undelayable. There’s a first rule to respect in such cases: The hypothesis of alternate paternity must be initially discussed with the mother alone. The physician, or the genetic counsellor, may ask the mother for a separate appointment to which she may come alone and during which the topic of alternate paternity may be discussed quietly. In any case, whether the mother wants to participate in such a preliminary, reserved discussion or not, there are several plausible options that must be discussed with the couple to explain the incongruity in the family testing results. Let’s assume that the child is confirmed to be affected by an autosomal recessive disorder caused by two mutations, of which one is found in the mother but the other one is not found in the father. Such negative results in the father may be caused by one of the following situations:
- sample exchange in the lab
- mistakes in lab reaction preparation
- use of decayed reagents (typically decayed or aspecific primers)
- problems with lab machinery (e.g., temperature retention of the thermocyclers)
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