[Human-animal chimeras. A systematic review of the ethical problems they raise and their possible solutions.]

Human-animal chimera research has gradually evolved to the present day, in which large projects related to the attempt to solve pathologies that help us human beings to alleviate diseases. However, it must be considered that many of these advances in science imply an important ethical dilemma in many cases, and even more so if we involve people in said experiments. In the present systematic review we sought to identify these ethical problems related to chimeras, as well as possible solutions to them proposed in the literature, including technical means for the realization of less humanized chimeras. A bibliographic search was carried out in the Pubmed, Embase and Medes databases on January 4 th, 2022. The articles that strictly comply with the objectives selected for the completion of the work will be selected. A total of 21 articles makes up our sample, from which ethical problems related to chimeras, possible solutions and technical means to avoid obtaining too humanized chimeras will be extracted. The issues identified in the articles are problems related to animal welfare, acquisition of human traits from chimeras, medical concerns derived from experimentation such as zoonoses, the origin of pluripotential cells for chimera production, the creation of human gametes by said chimeras, neurological chimerism and the moral status of chimeras. This paper provides solutions for these problems, such as the use of suicide genes in human cells that would be activated if they differentiate into neuronal cells or the use of gene editing through the CRISPR/Cas9 mechanism to incapacitate these cells so that they do not differentiate into neuronal cells. The only question that remains elusive to the proposal of solutions is the one related to the potential moral status of chimeras. It is certainly a complex issue given the variety of proposals on the concept of moral status available in literature. It is therefore necessary to bring these proposals closer to reflection on human-animal chimeras in order to initiate a discussion that can shed light on this issue.