Indivisible By Two Nancy L. Segal, 2005 253 Pages

Indivisible By Two. Nancy L. Segal. 2005 253 pages.

As the grandfather of fraternal twins, with a lifelong interest in the nature/nurture debate, I read this somewhat outdated book hoping for some insights. This is the second book about twins by this twin Director of the Twin Studies Center at California State University, Fullerton, formerly at the Twin Studies Center at the University of Minnesota. The twelve stories are all interesting in themselves, but are almost all about identical twins, triplets or one set of four, i.e. two sets of identical twin boys from one pregnancy. They do little to add to the extant twin studies that attempt to determine the relative contributions of genetics and environment to physical and mental characteristics. Most twin studies addressing this compare the frequency of specific traits in identical (monozygotic) twins compared to fraternal (dizygotic) twins. In addition, the environmental influences can be suggested by comparing identical twins reared together vs those reared apart, not only with respect to normal behaviour, but also to specific disease susceptibilities.

Several of these stories feature cases of mistaken identity that would make Shakespeare envious. And some highlight the uncertainties and mistakes of adoption systems and bizarre legalities around the world. An identical twin man had to legally adopt his genetic son because that son was carried to term after artificial insemination of his wife’s identical sister as the surrogate mother. With several adoptees in my extended family, the adoption stories were of interest to me. A story of an adoption agencies’ errors in Ottawa gave the book some local colour for me.

There are specific interesting observations that are difficult to explain on a purely genetic or purely environmental basis. Why is only one of three identical triplet men a homosexual, while the other two both have multiple sclerosis, albeit at different stages? (The gay one may also have this affliction, with early symptoms.) Even more confusing is the pair of identical twins, only one of whom is a transsexual. And when identical twins marry identical twins and then have identical twins, the labels can get problematic. Your uncle is then genetically your father, a your mother is genetically your aunt, and your brother is genetically indistinguishable from your father and your uncle.

This book does not even mention the word ‘epigenetics’, a term that came into use in the last 15 years. This refers to the variable influence of environmental exposures to the expression of specific genes; in the past the variability of the result of a gene’s action in different individuals was called ‘variable penetrance’ covering up the true meaning: “We don’t know why this happens”. Epigenetics studies have the potential to explain away much of the discrepancies in the nature/nurture debate, and shed a lot of light on past twin studies.

One does not need a science background to enjoy these twelve stories of human development. In fact, I am not sure it even helps. They are great human interest stories on their own.