Identical twins’ shared DNA has led to some extraordinary dilemmas when it comes to assigning paternity or assigning blame.  For example, I am aware of several cases in which women have had sexual relations with both members of an identical males twin pair, such that the biological father of their children cannot be identified.  Strangely, the identity of the child’s maternal grandmother (the twins’ mother) is certain! Such situations can place unusual strains (and limits) on legal decision-making.

A different sort of case poses the same uncertainties. There was a recent robbery at Kaufhaus des Westens (KaDeWe), a famous Berlin department store. KaDeWe happens to be the second largest store in Europe after Harrod’s.  Watches and jewelry, worth up to 100,000 euros, were stolen.  However, some clues were left behind. A rope ladder and a glove provided investigators with DNA that they hoped would help them find the three suspects. It turned out that one DNA sample was linked to two people—identical twins, Abbas and Hassan O. Now each twin can claim that his twin brother had committed the robbery.   It will be fascinating to follow this case as it unfolds.

Strictly speaking, identical twins may not share 100% of their DNA.  For example, co-twins may show copy number variations (CNV), i.e., altered DNA sequences that can happen due to mutatiosn, deletions or other processes. CNVs may be responsible for some inherited diseases, so they are a focus of great research interest at the moment.  However, the technology to identify a twin father or a twin criminal by means of CNVs is currently unavailable.

There are other relevant situations of interest. One member of an identical male twin pair was born with anorchia, a condition in which one or both testicles are absent at birth. However, the co-twins agreed to a testicle transplant—and the recipient twin was able to father children.  Ovary transplants have also occurred successfully between identical twin women. Parents of such children would be genetically indistinguishable. In fact, when ordinary identical twins marry and have children, the twin parent and twin uncle or aunt share the same genetic connection to their child and niece or nephew.  Perhaps the most gratifying case was documented in my recent book, Indivisible by Two. An infertile female twin became a mother twice—because her twin sister was artificially inseminated with her brother-in-law’s sperm.  The children that resulted were as biologically related to their parents as if they had conceived them naturally.

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