If identical twin brothers are each potentially the father of the same child, is there a way to determine, genetically, which is the father?
Antoon Pelsser
Maastricht, The Netherlands
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In a , a person suspected of rape claimed that his identical twin brother was the perpetrator. A careful DNA analysis revealed a few differences in the DNA between the brothers. And the DNA profile of the suspected twin matched with that found on the victim.
Gabriel N茅ve
Aix-Marseille University, France
From one generation to the next, there are always some small changes, called mutations, among the 3 billion or so base pairs of the human genome.
In order to identify which identical twin is the father, you would need to identify the mutations that ended up in their reproductive tissue, or germ line, and which occurred soon after the fertilised egg split into two, going on to develop into the twins.
On average there are between 10 and 20 such mutations between identical twins. To identify these, one would need to get samples of semen from both twin brothers, and tissue samples from the child and its mother and then obtain high quality genetic sequences from all of these.
Even so, it isn鈥檛 always possible to identify which of the brothers is the father, as in one-tenth of such cases, no mutations occurred in the early development of the twins, but, in the most favourable instances, there might be more than 100 mutations. In the case of identical triplets, some mutations may affect two of the brothers and not the third one.
Derek Abbott
University of Adelaide, Australia
It is possible to determine the genetic difference between identical twins if you target single nucleotide polymorphisms (SNPs) in their DNA rather than the short tandem repeats (STRs) that are used for standard DNA tests. SNP analysis is much more fine-grained, as each SNP represents a difference in a single nucleotide, the building block of DNA.
A method to solve paternity cases involving identical twins as alleged fathers using SNPs is outlined in a 2014 paper .
I also used SNP analysis to solve the 鈥淪omerton man鈥� mystery 鈥� the case of a man found dead in 1948 on Somerton beach in Adelaide, whose identity had never been discovered.
In 2012 I realised that some of this man鈥檚 hair was stuck in a plaster cast that had been made of his head, and, working with geneticists, tried to extract DNA from this hair. We tried a hair root first and it was a big flop. But we got 2 million SNPs from the hairshaft. Standard forensic wisdom is that the hair shaft is useless, however that viewpoint evolved from the world of standard crime DNA techniques that use STRs, not SNPs.
Those SNPs were used to find distant cousins of the Somerton man on genealogical DNA databases. We took the closest SNP match and built a family tree of that person. This tree contained 4000 people and on that we found a man called Charles Webb whose death had never been registered.
We then looked for more of Webb鈥檚 closest living relatives (we found five of them) and tested their SNP overlap with DNA from the hair. They were all in the right range and so this confirmed that Charles was the Somerton man.
Sandy Lukats
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Try the judgement of Solomon.
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