Dr. Machin is a fetal/placental pathologist at Mount Sinai Hospital in Toronto, Ontario, Canada; and a part-time clinical Professor at the University of Toronto, Toronto, Ontario, Canada. He has published over 100 papers, many book chapters, and co-authored the Atlas of Multiple Pregnancy. His principal interests include twin pregnancy and pathology, perinatal pathology, genetics of Wilms tumor, and the etiology of hydrops fetalis. Currently, he resides with his wife of forty-four years; and enjoys choral singing, gardening, and spending time with his two sons and grandson.
Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review†
Version of Record online: 10 APR 2009
Copyright © 2009 Wiley-Liss, Inc.
American Journal of Medical Genetics Part C: Seminars in Medical Genetics
Special Issue: The Genetics of Twinning: From Splitting Eggs to Breaking Paradigms
Volume 151C, Issue 2, pages 110–127, 15 May 2009
How to Cite
Machin, G. (2009), Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review. Am. J. Med. Genet., 151C: 110–127. doi: 10.1002/ajmg.c.30212
How to cite this article: Machin G. 2009. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review. Am J Med Genet Part C Semin Med Genet 151C:110–127.
- Issue online: 23 APR 2009
- Version of Record online: 10 APR 2009
- monozygotic twins;
- twin studies
Monozygotic twins (MZ) are rarely absolutely “identical.” This review discusses the types of genetic/epigenetic and prenatal environmental post-zygotic mechanisms that cause discordance within such twin pairs. Some of these mechanisms—ranging from heterokaryotypia to skewed X-chromosome inactivation—may cause extreme discordance, but these extremes are merely the more emphatic examples of discordance that, to some degree, underlies the majority of MZ twin pairs. Because of the entrenched misconception that MZ twins are necessarily identical, many MZ twin pairs are mistakenly designated as dizygotic (DZ). Clinical benefits to accurate zygosity determination include correct solid organ transplantation matching, if one twin requires donation for a non-genetically mediated disease; the opportunity of preventive management for disorders that do not manifest synchronously; and better counseling to parents regarding their individually unique, and often psychologically puzzling, twin offspring. In twin pairs with complex and confusing biological origins, more detailed zygosity testing may be required. For example, intermediate trigametic and tetragametic chimeric dizygotic twins are reviewed, some of whom are, nevertheless, monochorionic (MC). Because of inter-fetal vascular anastomoses in MC twins, genetic results from blood samples may not accurately reflect discordance in solid organs. Previously, it was thought that MZ twinning was some sort of embryological fluke. However, familial monozygotic twinning is more common than suggested by the literature. Seven new families are presented in an accompanying paper. Despite the difficulties and dangers of twin pregnancy (especially so for MC twins), human twinning persists, and continues to both challenge and fascinate parents, clinicians and geneticists. © 2009 Wiley-Liss, Inc.