Is there a possibility that there is a person with exactly the same DNA as me?

Only clones and monozygotic twins have identical DNA, so I would say their probability estimate is based on how accurate the DNA test is and how airtight the connection between the semen sample and the crime is.

No, I think it means their tests are not reliable enough to rule out the possibility of a false positive.

I believe what they are talking about in the show is that the sample might be incomplete or the comparison method might not be 100% accurate.

It might be possible that there is an identical twin your parents have never told you about that is out there somewhere in the world ;-) And perhaps a one in ~infinity chance that someone has the exact same DNA coincidentally, but in the odds of that are very, very, very (etc.) close to zero.

No, there is not.

Don’t they usually just look at a few genes or portions of chromosome? This is far less work than comparing entire genomes, but far more likely to yield near-identical sequences, I would think.

The thing is that forensic science isn’t as powerful a tool as CSI leads many people to believe.

If you have an identical twin, then yes. The odds of every bit of your NDA matching a stranger’s DNA perfectly are probably about like winning the MegaMillions Lottery at every drawing for a whole year. But be aware than DNA matching for criminal cases looks at only a few specific loci and the alleles associated with them, so the chances of two separate people matching come in somewhere around 1 in 7,000. A DNA match alone should not be enough to convict someone, even though prosecutors often knowingly withhold that exculpatory evidence and falsely claim it is. Here’s a good discussion of this.

In a city of 7 million, with a 1 in 7.000 chance of a match, that would mean as many as 1,000 people would have matches. To establish guilt, prosecutors should also present a strong case exclusive of the DNA. They need to establish things such as motive, opportunity, method and non-DNA forensic evidence.

Thanks for a nice link, @ETpro.

It’s possible even without clones or monozygotic twins. But the likelihood is extremely low. Human DNA consists of more than 3 billion base pairs. Another person with exactly your DNA is like winning the lottery every week for the next hundred years.

@gasman You’re welcome.

@mattbrowne Thanks for backing me up.

Forensic DNA testing uses only a few easy to measure markers to match samples.

The small number of individuals who might have identical markers (excluding twins and clones) would still still not have identical DNA if their entire gene sequences were to be compared.