# DNA **URL:** \ **Category:** Factual Questions\ **Created:** [January 24, 2009, 4:52pm UTC](https://boards.straightdope.com/t/dna/482799 "2009-01-24T16:52:04Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![copperwindow](https://avatars.discourse-cdn.com/v4/letter/c/4af34b/32.png) [@copperwindow](https://boards.straightdope.com/u/copperwindow)\ **Post date:** [January 24, 2009, 4:52pm UTC](https://boards.straightdope.com/t/dna/482799/1 "2009-01-24T16:52:04Z")
If no two people have the same DNA, why do they say in court the the chances of someone else having this DNA are 1 in however many billion? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [January 24, 2009, 5:02pm UTC](https://boards.straightdope.com/t/dna/482799/2 "2009-01-24T17:02:12Z")
They don’t test every bit of the entire genome. That would take too long and be too expensive. The tests are done on smaller pieces that are good markers for the purpose. > **[DNA Testing: An Introduction for Non-Scientists](http://www.scientific.org/tutorials/articles/riley/riley.html)** > [@](#): > > There have been two main types of forensic DNA testing. They are often called, RFLP and PCR based testing, although these terms are not very descriptive. Generally, RFLP testing requires larger amounts of DNA and the DNA must be undegraded. Crime-scene evidence that is old or that is present in small amounts is often unsuitable for RFLP testing. Warm moist conditions may accelerate DNA degradation rendering it unsuitable for RFLP in a relatively short period of time. > > PCR-based testing often requires less DNA than RFLP testing and the DNA may be partially degraded, more so than is the case with RFLP. However, PCR still has sample size and degradation limitations that sometimes may be under-appreciated. PCR-based tests are also extremely sensitive to contaminating DNA at the crime scene and within the test laboratory. During PCR, contaminants may be amplified up to a billion times their original concentration. Contamination can influence PCR results, particularly in the absence of proper handling techniques and proper controls for contamination. > > PCR is less direct and somewhat more prone to error than RFLP. However, PCR has tended to replace RFLP in forensic testing primarily because PCR based tests are faster and more sensitive. That site goes on to explain PCR and RFLP testing in more detail that should help you. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [January 24, 2009, 5:14pm UTC](https://boards.straightdope.com/t/dna/482799/3 "2009-01-24T17:14:09Z")
> [@copperwindow](#): > > **If no two people have the same DNA** , why do they say in court the the chances of someone else having this DNA are 1 in however many billion? Aside from what **Exapno Mapcase** said, identical twins do, in fact, have the same DNA. That’s why they’re identical. This would also apply to identical triplets, quadruplets, etc. ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [January 24, 2009, 5:28pm UTC](https://boards.straightdope.com/t/dna/482799/4 "2009-01-24T17:28:15Z")
> [@Broomstick](#): > > Aside from what **Exapno Mapcase** said, identical twins do, in fact, have the same DNA. That’s why they’re identical. This would also apply to identical triplets, quadruplets, etc. But each would have different random mutations, so probably not 100% exact, right? ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [January 24, 2009, 5:39pm UTC](https://boards.straightdope.com/t/dna/482799/5 "2009-01-24T17:39:28Z")
> [@RaftPeople](#): > > But each would have different random mutations, so probably not 100% exact, right? Well, some random mutation in one somewhere is possible. You oculdn’t easily check, however. Likewise, there’s no absolute reason that two unrelated people couldn’t have the exact same DNA… but odds are that’s never happen and will never happen. ---
**Author:** ![copperwindow](https://avatars.discourse-cdn.com/v4/letter/c/4af34b/32.png) [@copperwindow](https://boards.straightdope.com/u/copperwindow)\ **Post date:** [January 24, 2009, 5:49pm UTC](https://boards.straightdope.com/t/dna/482799/6 "2009-01-24T17:49:44Z")
> [@Exapno\_Mapcase](#): > > They don’t test every bit of the entire genome. That would take too long and be too expensive. The tests are done on smaller pieces that are good markers for the purpose. > > [DNA Testing: An Introduction for Non-Scientists](http://www.scientific.org/tutorials/articles/riley/riley.html) > > That site goes on to explain PCR and RFLP testing in more detail that should help you. Do we know of any two people who are not twins that have the same DNA? ---
**Author:** ![Sofis](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@Sofis](https://boards.straightdope.com/u/Sofis)\ **Post date:** [January 24, 2009, 5:51pm UTC](https://boards.straightdope.com/t/dna/482799/7 "2009-01-24T17:51:58Z")
> [@RaftPeople](#): > > But each would have different random mutations, so probably not 100% exact, right? Not really. Mutations that happen in your body do not spread to the rest of the cells in your body. Any mutation that is present in all the cells of one’s body actually occured in one of the gonads of one’s parents. While a given cell from one twin may have a mutation not present in a cell from the other twin, the same thing is just as likely to happen with two cells from the same body. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [January 24, 2009, 5:58pm UTC](https://boards.straightdope.com/t/dna/482799/8 "2009-01-24T17:58:43Z")
> [@copperwindow](#): > > Do we know of any two people who are not twins that have the same DNA? No. And, although it’s numerically possible, the odds against it are truly astronomical. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [January 24, 2009, 6:08pm UTC](https://boards.straightdope.com/t/dna/482799/9 "2009-01-24T18:08:08Z")
> [@copperwindow](#): > > Do we know of any two people who are not twins that have the same DNA? Sort of a followup question - is there any person or persons who we actually know all about their DNA, or at least some version of their full genome, not worrying about mutations? Maybe a volunteer donor for the human genome project? ---
**Author:** ![PoorYorick](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@PoorYorick](https://boards.straightdope.com/u/PoorYorick)\ **Post date:** [January 24, 2009, 6:40pm UTC](https://boards.straightdope.com/t/dna/482799/10 "2009-01-24T18:40:41Z")
> [@Q.E.D](#): > > No. And, although it’s numerically possible, the odds against it are truly astronomical. [Defense lawyer]So you’re saying that it _is_ possible, correct?[/Defense lawyer] A common defense tactic. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [January 24, 2009, 6:54pm UTC](https://boards.straightdope.com/t/dna/482799/11 "2009-01-24T18:54:20Z")
> [@PoorYorick](#): > > [Defense lawyer]So you’re saying that it _is_ possible, correct?[/Defense lawyer] > > A common defense tactic. What’s it to you, if it’s possible? 😉 Personally, I think that if we’re going to prosecute people based on genetic statistics, the defense should have every right to harp on the fact that statistics are not certainties. It might be the only defense people have against co-incidence. Of course, that requires a mathematically literate jury I realize… 😃 ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [January 24, 2009, 6:54pm UTC](https://boards.straightdope.com/t/dna/482799/12 "2009-01-24T18:54:38Z")
> [@chrisk](#): > > Sort of a followup question - is there any person or persons who we actually know all about their DNA, or at least some version of their full genome, not worrying about mutations? Maybe a volunteer donor for the human genome project? [Craig Venter: The Diploid Genome Sequence of an Individual Human](http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0050254&ct=1). ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [January 24, 2009, 8:05pm UTC](https://boards.straightdope.com/t/dna/482799/13 "2009-01-24T20:05:39Z")
> [@Sofis](#): > > Not really. Mutations that happen in your body do not spread to the rest of the cells in your body. Any mutation that is present in all the cells of one’s body actually occured in one of the gonads of one’s parents. While a given cell from one twin may have a mutation not present in a cell from the other twin, the same thing is just as likely to happen with two cells from the same body. I was thinking of when the fertilized egg first splits. I was assuming errors are possibly introduced at that point. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [January 24, 2009, 8:15pm UTC](https://boards.straightdope.com/t/dna/482799/14 "2009-01-24T20:15:38Z")
> [@RaftPeople](#): > > I was thinking of when the fertilized egg first splits. I was assuming errors are possibly introduced at that point. Possible, but so far no one has actually documented that. There likely wouldn’t be much difference, possibly too little for our current techniques to pick up. ---
**Author:** ![Markxxx](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@Markxxx](https://boards.straightdope.com/u/Markxxx)\ **Post date:** [January 24, 2009, 8:33pm UTC](https://boards.straightdope.com/t/dna/482799/15 "2009-01-24T20:33:03Z")
Statistics don’t tell you what IS possible, they only tell you how LIKELY that possiblity is to happen. For instace, if you told me there is a one in a billion chance that sounds like a long shot, untill you realize there are 6.7 BILLION people in the world so that means statistically at least 6 people should have that (whatever it is) The thing about statistics is they only tell you what is LIKELY to happen. If I throw a coin in the air 100 times, statistically it should come up heads 50 times and tails 50 times. But if it came up heads 100 times, you would probably think the coin was fixed. But there is nothing that says it CAN’T come up 100 times head, only that it’s not likely to. Twins don’t have identical DNA, let’s face it nothing in the world is TOTALLY similar. The thing is it is VERY expensive and VERY time consuming to test for it. So much of their DNA is identical it could a lot of time and money to find the unidentical area. The thing is over time genes tend to mutate in ones body and expressors of those genes are turned on and off. NOVA had a great program on DNA and it compared the genes and the hormones that act on them. This could explain why one identical twin is gay and one is straight. (I am just using that as an example, there is no proof so far of expressors being responsible for gay or straight people) It showed identical twins as babies were identical and by the time you hit old age the expressors acting on those genes made the DNA much different as time goes by. Remember not only do you have genes but there are hormones acting on those genes that turn them on and off during your life and they don’t get activated at exactly the same time. This would cause varience in twins who are identical. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 24, 2009, 8:45pm UTC](https://boards.straightdope.com/t/dna/482799/16 "2009-01-24T20:45:41Z")
> [@](#): > > For instace, if you told me there is a one in a billion chance that sounds like a long shot, untill you realize there are 6.7 BILLION people in the world so that means statistically at least 6 people should have that (whatever it is) And just how many of those 6 or 7 people live in the victim’s hometown and managed to draw enough suspicion that the police even bothered to test them? If you’re going to take the view that statistics can never tell you anything certain, then _nothing_ can ever tell you anything certain, because all the knowledge we have of anything is purely statistical. > [@](#): > > Remember not only do you have genes but there are hormones acting on those genes that turn them on and off during your life and they don’t get activated at exactly the same time. And where do you think those hormones come from, eh? ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [January 25, 2009, 12:55am UTC](https://boards.straightdope.com/t/dna/482799/17 "2009-01-25T00:55:51Z")
> [@Chronos](#): > > And where do you think those hormones come from, eh? They are influenced by internal and external factors so even identical twins won’t produce the same levels of various hormones throughout their lives. ---
**Author:** ![MOIDALIZE](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@MOIDALIZE](https://boards.straightdope.com/u/MOIDALIZE)\ **Post date:** [January 25, 2009, 1:10am UTC](https://boards.straightdope.com/t/dna/482799/18 "2009-01-25T01:10:16Z")
The statistical probabilities are based on analysis of a sample population. We obviously don’t have a sample of the DNA of every person in the U.S. The FBI’s [CODIS](http://en.wikipedia.org/wiki/Combined_DNA_Index_System) database has over 4.5 million DNA profiles identifying 13 markers, so you can draw some pretty good statistical conclusions when comparing an individual’s DNA to the database. The makeup of the database is obviously key. You probably wouldn’t be able to draw a good statistical conclusion on the DNA of a person of European ancestry if you only have 6 alleles identified and the database you’re comparing the sample to consists of 200,000 DNA samples from Hispanics. That’s why I support increased sample collection in the U.S. I think the privacy concerns are largely overblown and more samples means more accurate comparisons and a means of exonerating the falsely accused. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [January 25, 2009, 1:32am UTC](https://boards.straightdope.com/t/dna/482799/19 "2009-01-25T01:32:05Z")
running calculations… ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [January 25, 2009, 1:34am UTC](https://boards.straightdope.com/t/dna/482799/20 "2009-01-25T01:34:43Z")
never mind [Next page](https://boards.straightdope.com/t/dna/482799.md?page=2)