A Tetragametic Chimera is an extremely rare situation in which two different zygotes have become one, by one’s being absorbed into the other. In effect, it means that the one life form left will have two different sets of DNA. Now, the theological problem arises when one realizes what the commitment to the human soul existing at the moment of conception commits one to saying that, in the case of Tetragametic chimerism, either one biological organism has two corresponding immaterial souls, or else that at least one person died at an extremely early stage after conception. I think the suggestion that there are two souls corresponding to one human body is simply misconceived and requires a Cartesian, rather than Hylomorphic, idea of the soul and it’s relation to the body. Certainly it would be unacceptable on a Catholic theology. However, the alternative seems grim: it entails that a human person died in the process of coming to term from a zygote. Is this a problem? Well, it seems to me that it is no more a problem than a miscarriage. It certainly isn’t a conceptual problem, and so poses no inconsistency for philosophical theology, and moreover it isn’t even as emotionally trying as miscarriages are, since at least with Tetragametic Chimerism there remains a living person who is brought to term. This is an example of natural evil.
What is interesting, I think, is to consider that it could be much more common than it is. It is extremely rare in human reproductive processes, but not nearly as rare in rodents like mice. However, it isn’t difficult to imagine a logically possible world in which the ratio of human Tetragametic Chimerism was greatly increased, and posed an emotional problem for people with respect to accepting the Catholic doctrine of the soul’s existence from the moment of conception. As it stands, it is a rather esoteric objection rarely brought up, but interesting to think through.
Finally, if somebody wishes to use the curious case of Tetragametic Chimerism against the Catholic doctrine that the soul exists from the moment of conception, then not only can the Catholic explain that this poses absolutely no conceptual problem (being an instance of natural evil) to the commitment to the soul’s real existence from the moment at which conception occurs (when some organism is no longer identifiable with it’s parents, or as a part of either of its parents, from a biological perspective), but the Catholic can go further. The Catholic can make the issue into a moral argument against in vitro fertilization since the likelyhood of Tetragametic Chimerism is significantly increased in in vitro fertilization.
The curious case of Tetragametic Chimerism also presents an interesting thought about sexuality, since sometimes the human Chimera has both male and female DNA. What, then, are we to say about such a person – are they male or female? Could they be ordained to the Catholic priesthood? I think if this makes it obvious that maleness and femaleness are accidental features of human persons, and not determined by substantial form (in other words, souls are not ‘naturally’ male or female). Of course, for the person who thinks they are, they could simply say that the original two zygotes were two persons, one male and another female, and whichever absorbed the other is the person who survived – even if we cannot know which one absorbed which. For prudential reasons the person who believes that souls are naturally either male or female would want to presume that human Chimera’s cannot be ordained. However, I think such thought experiments demonstrate both that there is no coherent way to make sense of maleness of femaleness as natural features of the substantial form corresponding to individual souls, and that things like the sacrament of ordination preclude women’s ordination by reason of the form of the sacrament rather than the matter (since the matter does not require a person who naturally is male if there is no such thing, but rather the form requires a person who is male by physical disposition).
I take it I’ve exhausted the interesting dimension of Tetragametic Chimerism in relation to Catholic theology and apologetics.