Paternity testing of children of identical twins...

Reading White Teeth, I am now wondering this: If a woman has sex with identical twins - say one night apart - and she becomes pregnant, would it be possible to determine paternity by examining the child’s DNA? Could a paternity test point to one but not the other as the father?


It has happened in real life.

Well… it would be extremely difficult.

You would have to test extensively to identify some of the slight genetic differences between the twins which arose due to mutation. Only upon identifying such faint differences could you differentiate paternity. Not easy. Not fast. Not practical.

Due to differences in inheritable gene expression, epigenetic testing theoretically will be able to id the actual parent in the future. Just not there yet.

I recall reading about paternity testing way back when. If the girl had the temerity to sue for paternity, the guy would threaten to ruin her reputation by having a dozen of his friends all testify they had sex with her too at the right time. Since blood type testing would only eliminate a few potential candidates, it was not too decisive.

Apparently, the story goes, one clever guy got his friends to testify they could be the father too. The judge had seen this trick, and ruled that was just fine, if they were all candidates, they could all pay their share of child support.

I assume this was the end of a dozen good friendships.

The differences would need to occur between the two brothers’ germ cells (cells in the testes that produce sperm). There’s likely to be nearly as much diversity among germ cells in one individual as between them, so there would be a fair bit of luck involved.

I believe the mutation rate is 1 per 100,000 divisions, in general, and it might be lower for the germ cell line. 100K generations produces a LOT of cells (in theory up to 2^100K but obviously not in practice.) How many divisions are there between conception and the adult germ cells? Probably far fewer, meaning there’s a good chance their DNA sequences are identical. If there’s only one mutation between the twins, chances are 50% it wouldn’t appear in any given sperm cell.

A twin in Spain, I think, was charged a while ago with a big robbery. Part of the evidence was recovered DNA, which, with all other solid evidence, the prosecutor thought sealed the case. The defendant was judged not guilty (or on appeal, perhaps) because it could not be proven that his identical twin brother didn’t commit the crime. I think the twins operated together on a number of crimes, aware of this protection.

Identical twins are genetically the SAME PERSON. They are natural clones, sharing 100% of their genes. Even if there is any offspring from such relationship, the son/daugher would still be genetically your OWN son/daughter, your twin is essensially ANOTHER COPY OF YOURSELF.

Twins share a higher kinship ratio (1.0) than even one’s own parents (0.5) or son/daughter (0.5). Funny how people freak out about human cloning when there are natrual clones around.

I know. I don’t like clones. They make me twitchy.

Definitely would have to look for mutations which occurred in the progenitor cell line that developed into a testes. But there are around 3 billion DNA base pairs subject to point mutation (most of which would be silent) in humans.

But we all pick up about 100-200 new mutations, which occur at a rate of about 1 in 15 to 30 million base pairs.

Tests which are currently commonly used (RFLP analysis) for paternity testing wouldn’t work to flesh out these differences. You would need extensive (think a full mapping of each twin’s DNA) testing to find these single nucleotide polymorphisms.

Find the difference between the twins and then test the kid to see which variant he has. Do that for a handful of the identified SNPs and you’ve established paternity.

Currently such testing would be cost and time limited. But it is theoretically possible.