We know that some cases exist in which two eggs are fertilized inside a woman quite naturally, and the genetically different cells divide but then touch each other and grow together. In some cases, one absorbs the other; this can lead to an individual with a partially formed fetus, or parasitic twin, inside or attached to him or her.
In still other cases, the two develop largely separately, but result in births of what were historically called Siamese or conjoined twins. Medical responses in developed countries usually call for separating the two, even when this means that one will die to allow the other to live, especially when the two share vital organs.
In perhaps the biologically most intriguing cases, the two combine more completely. Two different lines of cells, with different genes on different chromosomes, come together and, with the amazing regulatory powers of life, merge together to make a whole, apparently normal and natural chimeric person. The case of Karen Keegan brought chimerism to medical attention in 1998, when her sons were being tested as possible kidney donors for her. They failed to match her DNA in ways that suggested that they could not be her sons, yet she (and her family) knew they were. Doctors finally discovered that she had two distinct sets of DNA, which proved that chimerism does occur and yet remain invisible.
Another case—this one in 2002—involved Lydia Fairchild, who similarly failed to match her children genetically. This biological surprise led to court battles as Fairchild separated from her husband and sought child support. She also turned out to be a genetic chimera. The resulting 2006 British documentary The Twin Inside Me (or I Am My Own Twin) called into question using genetic testing as a reliable source of evidence about identity. Chimerism also challenges us to think more seriously about how we will handle “personalized” medicine for people who are actually complex mixes of genomic materials.
What does such developmental complexity mean for the proposed Sanctity of Human Life Act? If one twin absorbs part of the other, did it commit homicide? Perhaps so. Conjoined twins are considered to be two different people legally, but what if one of them has only legs and feet that stick out from the fully formed twin (which has actually happened)—is the one twin is a person with some extra parts, or did he or she also commit homicide in some way? What about a chimeric person—does he or she get two votes because of having come from two different fertilizations? If fertilization defines personhood, then surely so.
Clearly, this easily leads to nonsense. The Sanctity of Human Life Act legislation will almost certainly never become law, in part because the U.S. Congress is not doing much in these deeply divided times, and in part because members of Congress often propose such dead-end legislation to satisfy their core constituents and gain political and financial support. We know that. Nonetheless, the message is clear. For the 40 members of Congress who co-sponsored this bill, sanctity of human life is one of the top issues for gaining that political and financial support that they need so desperately. It helped place Paul Ryan as the conservative force on the Republican presidential ticket in 2012. Biological facts may not matter much to those constituents. But they should matter to otherwise intelligent people, which includes Paul Ryan and colleagues.
We can hope that our legislative, judicial, and executive governmental branches will learn enough biology so that they do not even consider legislation that makes little sense. Understanding what embryos are and how they develop is not just a theoretical matter—being inaccurate has consequences. Knowing the biology will not tell us how to act or what is right and good, but it will inform decisions so that they are not inconsistent with biological reality.
This article is part of Future Tense, a collaboration among Arizona State University, the New America Foundation, and Slate. Future Tense explores the ways emerging technologies affect society, policy, and culture. To read more, visit the Future Tense blog and the Future Tense home page. You can also follow us on Twitter.