Tag Archives: Embryology

Genetic Identity & Chimeras

Details of bony structures in 14-week-old fetus from an ultrasound equipment manufacturer.

If you’ve followed this blog for long, you know I am pro-life, both on religious grounds and on scientific grounds. In a recent discussion on Twitter, I made a point I have made before, that the fetus cannot be a part of the mother’s body (as many pro-abortion advocates say), partly because the fetus has different DNA from the mother. Later in the discussion, an objection was raised to this point that I hadn’t heard before, so let’s work through that today.

For those who may have been living under a rock the last 60+ years, DNA is the complex molecule Deoxyribonucleic Acid found in every living cell that stores the “blueprint” for that person. First discovered in 1869, it’s structure was finally determined in 1953, and the staggering informational content fully mapped in 2003. After that slow start, our knowledge about DNA and uses for that knowledge have increased dramatically over the years. Some of the most common uses of DNA testing include determining parentage, convicting guilty criminals (or exonerating those wrongly convicted), and identifying partial or unrecognizable remains. Essentially, these examples use DNA to verify the unique identity of individual persons. Now combine that with the well-established fact that by the time fertilization is complete (within 24 hours of the joining of sperm and oocyte), and while still only a single cell, a developing baby has DNA distinct from either parent. The obvious conclusion, biologically, is that this rapidly developing organism is not the same organism as the mother.

Now, the objection raised was that unique DNA doesn’t determine how many lives are present because of the existence of chimeras. What’s that, you ask? A chimera, outside of the mythological monster from which the name is drawn, is an organism with two (or more) distinct sets of DNA. Though not “new” in terms of existence, the first confirmation of a natural human chimera was in 1953 when a woman in England donated blood and it was found to contain two different blood types in one sample. As our knowledge of genetics has grown and DNA testing has become more commonplace, so too has observance of this phenomenon. for instance, a woman needed a kidney transplant in 1998, but when her 3 sons were tested as potential donors, 2 of them were determined, based on DNA, not to be her biological sons, even though she had given birth to them. Then in 2003, a woman in Washington filed for welfare benefits for her children and was denied, with accusations of welfare fraud pending, because her 2 children were determined not to be hers. A 3rd child was born while this was being investigated, so that birth and an immediate DNA test of both mother and child were witnessed by an officer of the state. Again, the DNA test showed different parentage for the child just born. What happened in each of these cases? Each of these 3 women had been twins. The Englishwoman in 1953 had a twin brother who had died shortly after birth. Cells had been shared between the two early in the pregnancy. The other 2 women were both the result of fused embryos, or a “vanishing twin”. Two oocytes had been fertilized by two sperm, resulting in twin zygotes. Early in the pregnancy, however, the two zygotes merged into one. Because they were separate zygotes, they each had different DNA. However, because this occurs very early in development, the zygote is still a collection of totipotent cells (meaning each cell at this stage can still become any cell in the human body, i.e. they have not differentiated into their separate lines of specialized cells for organ generation). When the twin zygotes (call them A & B) fused together, some of the cell from twin A went on to form various body parts like the skin cells inside the cheek where DNA samples are often taken. Cells from twin B went on to form other body parts such as the ovaries that would be responsible for producing children “not her own”. A third scenario, Fetal Microchimerism, or FMc, is much more common and is when cells from the blood of the fetus and/or mother get through the placental barrier to reside in the other person. Cells from their children have been found in the bodies of autopsied women many years after their pregnancies.

Based on these observed cases, we know that a person can have multiple DNA. But does the existence of chimeras refute the idea that the developing baby is a unique individual distinct from the mother? I don’t think so. After all, when a patient receives an organ transplant, the donated organ will have the donor’s DNA rather than that of the recipient; but nobody considers the donor and recipient to be part of the same body. Furthermore, even though the person may have 2 sets of DNA in their body, the transplanted organ is only one organ, and cannot become anything more. A zygote, on the other hand, is capable of developing into a mature human, and is, in fact, directing much of the pregnancy. The case of cells passing between twins in utero, as in the 1953 English case, is really no different than the case of organ donation between adults. The case of fused zygotes is more extreme in that all of the “donor” has been passed to the recipient, but the concept of a donor providing some portion of a recipient’s organs still applies. Because the transfer of genetic information occurs at such an early stage, it’s impossible to know which organs formed from donor and which from recipient without some kind of comprehensive test that is not practical at this point, but it’s important to remember that neither zygote had the same DNA as the mother, so the resulting chimera is still not part of the mother’s body no matter how you look at it. As far as fetal microchimerism, we are only talking about a few individual cells from a genetically unique human (i.e. the baby) passing through the barrier that normally separates the baby’s blood from the mother’s, and residing in the body of another genetically unique human (i.e. the mother). The fact that a few of the baby’s cells migrate into the mother’s body (and vice versa) no more make the baby part of her body than an organ donor’s cells inside a recipient’s body makes the donor part of the recipient.

Does the chimeric objection succeed? No. Even with individual persons not necessarily being limited to only one DNA in their body, the baby is at all stages of development a separate, self-contained organism temporarily residing in the mother for nourishment and protective environment, and not a “part” of her that just has different DNA. All cases of chimerism, both natural and artificially induced, come about from the involvement to one degree or another of a second, genetically distinct organism. The different DNA confirms this and actually bolsters our understanding of a baby as a genetically unique individual from conception.


Further reading: “The Human Chimera: Legal Problems Arising From Individuals with Multiple Types of DNA“, by Robert Russell Granzen, Seton Hall Law School, 2014, was a thorough and interesting read on the matter.