Genetic Differences Between Identical Twins

by Dr. Hsien-Hsien Lei
Posted February 20, 2008 in DNA in General

Identical (monozygotic) twins Raymon and Richard Miller had better watch out. The two were embroiled in a paternity suit that alleged both had slept with the same woman on the same day and nobody knew which brother had impregnated her. In May 2007, the judge decided that Raymon is the legal father of the child who was subsequently born although child support was split between the two brothers.

Standard paternity testing examines 16 DNA markers which is enough to make them over 99.99% accurate. In the case of the State of Missouri and Holly Marie Adams vs. Raymon and Richard Miller, the paternity test showed that the two brothers both had a 99.999% probability of being the father. There is currently no commercially available test that can determine which of the twin brothers passed his DNA to the child even though there are ways in which the genomes of identical twins differ.

Epigenomic chemical modifications. Researchers at Ohio State University found epigenetic changes in twins’ genomes that increased as pairs of twins aged. One of the main epigenetic processes that occurs to our DNA is methylation which can be caused by environmental exposures, such as diet and physical activity. Methylation can lead to differences in gene expression and as we age, the amount of DNA methylation increases. So, it’s expected that identical twins will grow less and less similar in their patterns of DNA methylation and gene expression as their lives progress especially if their lifestyle habits and surroundings differ greatly.

DNA copy-number-variation profiles. Another way in which the genomes of identical twins may differ is in copy number variation (CNV) that appears as segments of DNA that are missing, occur in multiple copies, or have flipped orientation in the genome. Identical (monozygotic) twins have been found to have different CNVs which could explain why even identical twins are not truly identical in appearance or other physical characteristics despite similar environmental exposures. For example, one twin sometimes develops a disease while the other does not. (HT: DNA Direct Talk)

What does this mean for Raymon and Richard Miller? DNA methylation patterns and copy number variations can be definitive in a paternity case if differences between the brothers are identified. Then, by comparing the same genomic regions in the child, it may be possible to see who the s/he more closely resembles genetically keeping in mind that the child is also accumulating his/her own DNA methylation and CNVs. While this type of genetic analysis isn’t currently available, it will most likely be available in the child’s lifetime. I would suggest that the Millers’ love child or his/her guardian store DNA from Raymon and Richard Miller for future analysis.

Update:: John Hawks has more.

Photo credit: Wellcome Images under Creative Commons.

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12 Comments »

Comment by Lisa

2008-02-20 18:30:55

The genetics of identical twins are in the news, indeed! (And I’ll bet on daytime TV talkshows, if the producers can talk them into it.)

You suggest that the Millers’ love child or his/her guardian store DNA from Raymon and Richard Miller for future analysis. Perhaps we should send them some DNA Archives, which use the new Biomatrica DNA SampleMatrix technology to store DNA safely, indefinitely. Then, their child has the *choice* whether or not to pursue more genetic information later, when testing technology is further developed.

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Comment by Hsien-Hsien Lei, PhD

2008-02-20 19:24:44

We should send them complimentary DNA Archive kits only if they agree to mention it on the air when they’re guests on Maury.

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Comment by Simon

2008-02-20 20:21:54

In the case of the State of Missouri and Holly Marie Adams vs. Raymon and Richard Miller, the paternity test showed that the two brothers both had a 99.999% probability of being the father

What a bizzare abuse of statistics - assuming they were truly twins (not a pair of siblings from triplets, quads etc.) then they both Raymon and Richard have a 49.9995% probability of being the father.

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Comment by Hsien-Hsien Lei, PhD

2008-02-20 20:34:27

Hi Simon, Thanks for the comment. You make a great point! It’s a bizarre case indeed.

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Comment by autumnmist

2008-02-21 04:01:36

Thanks for writing this. When I first heard of this story, I was kinda annoyed at the writeups because they were all claiming that it’s “impossible” to tell the difference between the two twins by genetics, but that’s totally not true–there’s just no commercial test available yet (as you pointed out).

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Comment by Hsien-Hsien Lei, PhD

2008-02-21 09:03:58

autumnmist, Nothing is truly impossible, wouldn’t you say?

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Comment by Barry

2008-02-21 05:32:49

Interesting post. I wonder if the changes you talk about will be found in all cells or just in a subset of each twin’s cells. If just a subset, then I suppose the differences you describe will need to happen in the sperm DNA the twin contributes to the child in order to be used in a paternity test.

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Comment by Hsien-Hsien Lei, PhD

2008-02-21 09:03:13

Hi Barry, It’s definitely a complicated situation. I was wondering about sporadic mutations as well whether they would be heritable and what samples from the twins would be tested.

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Pingback by DNA Direct Talk » Blog Archive » What We’re Talking About This Week

2008-02-22 19:13:02

[...] testing. And twins… Hsien Lei trumped my posts on twins (as always) with her discussion about a paternity suit involving identical twin brothers. She explains more about how there can be genetic differences between identical twins, who hatch [...]

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Pingback by Grand Rounds 4.22: The Future of Medicine « ScienceRoll

2008-02-26 18:44:13

[...] Hsien-Hsien Lei at Eye on DNA examined the genetic differences between identical twins. [...]

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Pingback by Interesting Links From The Genetic Genealogist » The Genetic Genealogist

2008-02-27 07:03:19

[...] to anyone who has known identical twins that are as different as night and day. See more at “Genetic Differences Between Identical Twins” at Eye on DNA, “Non-Identical Identical Twins” at John Hawks Weblog, and [...]

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Pingback by Eye on DNA Headlines for 11 April 2008

2008-04-11 20:22:26

[...] nitpicky point about the DNA fact that “identical twins have identical DNA”…erm, not quite [...]

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