Debates surrounding the psychological implications of human reproductive cloning (HRC) escalated in 1997, following the 1996 birth of Dolly, the cloned Scottish lamb. For the first time, human cloning was no longer a vague uncertainty but a real possibility. Aside from the physical risks to which cloned persons might be subjected, there was concern over psychological implications associated with family structure and relationships. Would cloned persons be deprived of autonomy and independence? Would parents impose unfair expectations on children who were their genetic replicas? Since then, purported attempts to clone a child1 have kept worries over HRC's societal consequences in the public consciousness.
Possible sequelae of HRC have been addressed in journal articles,2,3 books,4,5 edited volumes,6 legal symposia,7 newspaper and magazine articles,8,9 and scientific documentaries.10 The National Bioethics Advisory Commission presented an array of hypothetical consequences of HRC, many involving potential harm to cloned children and adverse effects on society.11
A review of the various sources reveals inappropriate reference to cloned children as "genetic twins" of their donors and neglect or misrepresentation of scientific twin research findings. This is unfortunate because twins and twin research offer the best available model (although not the only model) for informed evaluation of HRC's psychological effects. The present essay will address the question, "Who is a twin?" and examine the ways in which twin studies can clarify thinking about clone-donor resemblance, individuality, and social relationships. Some research designs for further addressing various concerns will also be proposed.
CRITERIA FOR TWINSHIP
Identical (monozygotic, or MZ) twins are clones (genetic replicas of another individual or organism), but clones are not identical twins. More formal definitions include "a plant, animal, or other organism that is genetically identical to its parent," or "a collection of organisms, cells, or molecular segments that are genetically identical, direct descendants of a single parent by asexual reproduction."12 It might be objected that MZ twins are genetically identical to each other, not to a parent, and so do not fit these definitions. However, if we consider that MZ twinning results when a zygote divides between the first and 14th postconceptional day,13 it is possible to consider the single zygote as the "parent" that gives rise to an identical copy.
Despite MZ twins' (and clones') direct descent from a genetically identical other, members of selected pairs may show some genetic differences. These are variously explained by postzygotic chromosomal nondisjunction, differential X-chromosome inactivation in female pairs, differential gene imprinting, and mutations for autosomal single-gene diseases.14 Nevertheless, MZ twins are more alike than any other pair of
individuals across all measured behavioral and physical traits, and even separately reared twins show striking similarities.
Establishing twinship criteria is important because of frequent reference to cloned children as "twins" or "delayed genetic twins" of their parents or other individuals.15 These terms convey concepts of sameness and intimacy, and could impose unfair expectations and demands upon individuals who do not fit the criteria. Four twinship criteria, including discussion of exceptional cases, are presented below.
A unique feature of MZ twins is that co-twins are conceived at the same time. In contrast, clones and their parent donors would be conceived years apart, and clones and their donor siblings would be conceived at least 9 months apart.
SHARED BIOLOGIC PARENTS
MZ twins share their parents in a biologic sense because their mother released the egg that was fertilized and she carried them throughout the pregnancy. Half the twins' identical genes come from their mother and half come from their father. However, donors and clones share their parents only in a technical sense. This is because, despite their identical genes, the donor's mother (the clone's grandmother) gestated the donor but not the clone. Furthermore, unless the donor gestated the clone created from one of her cells, the 2 would not share mitochondria. Mitochondria are passed down through families on the maternal side, and a clone created from a father would not share his mitochondria unless gestated by his sister or mother. However, not sharing mitochondria should have little effect on clone-donor similarity because mitochondria represent a very small fraction of the genome, and are more directly associated with cellular functioning than with phenotypic expression.
Twins develop in the same intrauterine environment, yet this environment is not truly the same for MZ twins. Approximately two thirds of MZ twins (those resulting from delayed zygotic division) are affected by mutual circulation in utero (fetal anastomosis) that can produce co-twin differences in size and health.13 Interestingly, some studies have shown greater intellectual and personality similarities among later-splitting twins than early-splitting twins.16 Donors and clones would share neither intrauterine environments nor prenatal events. However, adverse prenatal events are more closely tied to MZ co-twin differences, not similarities. It is, therefore, possible that a donor and a clone might be more physically alike in certain traits than MZ twins because each would have been gestated separately, unaffected by the adversities typical of multiple-birth pregnancies.
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