It is technically feasible to perform paternity diagnosis testing solely involving an alleged father and his descendent. However, there are serious legal and ethical problems for forensic genetics laboratories when it comes to paternity testing cases for investigating the alleged father-child relationship if the biological mother has not given consent to access her genetic information. Based on the Spanish Constitution, the new Code of Ethics of the Spanish Medical Association includes several articles on studies about genetic information and their acceptance by all the individuals involved. This problem is greater when the child is a minor, mentally incapacitated or psychologically incapable, because current Spanish law requires informed consent from legal representatives, but the law does not typify what happens when one parent gives consent (the putative father) and the other parent (the mother) does not agree. The aim of this study is to put forward legal solutions to avoid potential legal problems.