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This was a plot twist in an episode of CSI:Miami several years ago. Identical twins were implicated in a crime for which DNA was left behind, and they thought

This was a plot twist in an episode of CSI:Miami several years ago. Identical twins were implicated in a crime for which DNA was left behind, and they thought they were safe by being twins. But they also had distinct T cell (etc) signatures and that solved the case.

thfuran 4y ago
Due to mutations, identical twins probably don't even have identical DNA by the time they're born, let alone in their adult life. I don't think forensic DNA testing is a full sequencing though.
- jostmey 4y ago
  It is even more complicated than that! Some immune cells, like T cells, will actually edit their own DNA through a process named V(D)J recombination. So the immune system is actually editing some of its own DNA! https://en.wikipedia.org/wiki/V(D)J_recombination https://en.wikipedia.org/wiki/V(D)J_recombination
  - jjtheblunt 4y ago
    Like Lisp reader programming for dsl creation?
  - thfuran 4y ago
    Right, but I mean even of non immune cells.
robocat 4y ago
Human Chimeras[1] are even more weirder: one person with cells from two fraternal twins. Causes real problems with maternal[2] and paternal[3] DNA testing. Even more problems[4] if the twins are male and female. [1] https://duckduckgo.com/?q=human+chimera https://duckduckgo.com/?q=human+chimera [2] https://wikipedia.org/wiki/Lydia_Fairchild https://wikipedia.org/wiki/Lydia_Fairchild — Fairchild stood accused of fraud by either claiming benefits for other people's children, or taking part in a surrogacy scam, and records of her prior births were put similarly in doubt . . . [3] https://time.com/4091210/chimera-twins/ https://time.com/4091210/chimera-twins/ — the father had absorbed some of his twin's cells when he was a fetus, effectively becoming a chimera of himself and his brother. The man's previous child's DNA matched his . . . [4] https://wikipedia.org/wiki/46,XX/46,XY https://wikipedia.org/wiki/46,XX/46,XY Sex-chromosome discordant chimerism (XX/XY chimerism)
- genomer 4y ago
  Those are interesting cases. A similar but more ubiquitous strangeness is somatic mosaicism, specifically in the brain where it seems that "no two neurons are genetically alike." I was in line for the Terminator ride at Universal Studios when I stumbled onto this article a few years back. Couldn't stop thinking about it the rest of the trip. https://www.scientificamerican.com/article/scientists-surprised-to-find-no-two-neurons-are-genetically-alike/ https://www.scientificamerican.com/article/scientists-surpri... “The idea is something that 10 years ago would have been science fiction,” says biochemist James Eberwine of the University of Pennsylvania. “We were taught that every cell has the same DNA, but that’s not true.” Edit: Reading into this more again for the first time in a while. I'm amazed at how large some of these differences are. Many having 1 million base pair copy number variants. "Single cell sequencing of endogenous human frontal cortex neurons revealed that 13%-41% of neurons have at least one megabase-scale de novo CNV, that deletions are twice as common as duplications, and that a subset of neurons have highly aberrant genomes marked by multiple alterations." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975283/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975283/ Couple this with... "Megabase-scale copy number variants (CNVs) can have profound phenotypic consequences. Germline CNVs of this magnitude are associated with disease and experience negative selection. However, it is unknown whether organismal function requires that every cell maintain a balanced genome. It is possible that large somatic CNVs are tolerated or even positively selected." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772019/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772019/ I can't imagine our brains are just accumulating mutations of this size for no reason. My favorite theory so far is that it has to do with memory indexing. https://www.frontiersin.org/articles/10.3389/fgene.2020.00370/full https://www.frontiersin.org/articles/10.3389/fgene.2020.0037...

thfuran 4y ago

Due to mutations, identical twins probably don't even have identical DNA by the time they're born, let alone in their adult life. I don't think forensic DNA testing is a full sequencing though.

jostmey 4y ago
It is even more complicated than that! Some immune cells, like T cells, will actually edit their own DNA through a process named V(D)J recombination. So the immune system is actually editing some of its own DNA! https://en.wikipedia.org/wiki/V(D)J_recombination https://en.wikipedia.org/wiki/V(D)J_recombination
- jjtheblunt 4y ago
  Like Lisp reader programming for dsl creation?
- thfuran 4y ago
  Right, but I mean even of non immune cells.

robocat 4y ago

Human Chimeras[1] are even more weirder: one person with cells from two fraternal twins. Causes real problems with maternal[2] and paternal[3] DNA testing. Even more problems[4] if the twins are male and female. [1] https://duckduckgo.com/?q=human+chimera https://duckduckgo.com/?q=human+chimera [2] https://wikipedia.org/wiki/Lydia_Fairchild https://wikipedia.org/wiki/Lydia_Fairchild — Fairchild stood accused of fraud by either claiming benefits for other people's children, or taking part in a surrogacy scam, and records of her prior births were put similarly in doubt . . . [3] https://time.com/4091210/chimera-twins/ https://time.com/4091210/chimera-twins/ — the father had absorbed some of his twin's cells when he was a fetus, effectively becoming a chimera of himself and his brother. The man's previous child's DNA matched his . . . [4] https://wikipedia.org/wiki/46,XX/46,XY https://wikipedia.org/wiki/46,XX/46,XY Sex-chromosome discordant chimerism (XX/XY chimerism)

genomer 4y ago
Those are interesting cases. A similar but more ubiquitous strangeness is somatic mosaicism, specifically in the brain where it seems that "no two neurons are genetically alike." I was in line for the Terminator ride at Universal Studios when I stumbled onto this article a few years back. Couldn't stop thinking about it the rest of the trip. https://www.scientificamerican.com/article/scientists-surprised-to-find-no-two-neurons-are-genetically-alike/ https://www.scientificamerican.com/article/scientists-surpri... “The idea is something that 10 years ago would have been science fiction,” says biochemist James Eberwine of the University of Pennsylvania. “We were taught that every cell has the same DNA, but that’s not true.” Edit: Reading into this more again for the first time in a while. I'm amazed at how large some of these differences are. Many having 1 million base pair copy number variants. "Single cell sequencing of endogenous human frontal cortex neurons revealed that 13%-41% of neurons have at least one megabase-scale de novo CNV, that deletions are twice as common as duplications, and that a subset of neurons have highly aberrant genomes marked by multiple alterations." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975283/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975283/ Couple this with... "Megabase-scale copy number variants (CNVs) can have profound phenotypic consequences. Germline CNVs of this magnitude are associated with disease and experience negative selection. However, it is unknown whether organismal function requires that every cell maintain a balanced genome. It is possible that large somatic CNVs are tolerated or even positively selected." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772019/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772019/ I can't imagine our brains are just accumulating mutations of this size for no reason. My favorite theory so far is that it has to do with memory indexing. https://www.frontiersin.org/articles/10.3389/fgene.2020.00370/full https://www.frontiersin.org/articles/10.3389/fgene.2020.0037...