Your DNA is almost certainly yours alone. The only people who come close to sharing it are identical twins, and even they are not perfect copies, since science can now find the few spots where they differ.1PubMed. Differences between germline genomes of monozygotic twins The bigger surprise is that one person’s DNA isn’t fully uniform either. It shifts slightly from cell to cell, and some people even carry cells from someone else.
Why No Two People Share the Same DNA
Any two people share about 99.9% of their DNA. The leftover fraction still adds up to millions of letters, and scientists have catalogued over 88 million spots where human DNA varies from person to person.2PubMed. A global reference for human genetic variation Most of these are single-letter differences, about 84.7 million of them.3PubMed. A global reference for human genetic variation Most variants do nothing noticeable, but some affect traits like eye colour or disease risk.
Each child gets a random half of each parent’s DNA, and that is where the mixing happens. Every egg and sperm carries a different half. Each parent’s DNA comes in two copies, one from each of their own parents, and before an egg or sperm forms, those copies swap segments. So the half a parent passes on is a patchwork, not a clean copy of either grandparent’s DNA. Two full siblings share roughly half their variable DNA and still differ in millions of spots. That’s why full siblings can look alike but never share a full DNA profile. Only one event, an embryo splitting in two, produces two people from one mix.
Do Identical Twins Have the Same DNA
Identical twins come from one fertilized egg that splits in two. They start with the same DNA, but they don’t stay that way. On average they differ by about 5 small DNA changes, called mutations, that arose in early development.4PubMed. Differences between germline genomes of monozygotic twins About 15% of identical twin pairs have a substantial number of these changes found in only one twin.5PubMed. Differences between germline genomes of monozygotic twins The changes happen while the embryo is forming, and neither parent passes them down.6PubMed Central. A characterization of postzygotic mutations identified in monozygotic twins Some date to the earliest cell divisions, as early as when the embryo was just two or four cells.7PubMed Central. Functional landscape of genome-wide postzygotic somatic mutations between monozygotic twins
Twins also drift apart in how their genes are used. Chemical tags sit on top of the DNA and control whether a gene is read, and they can shift with age, diet, smoking and environment. Young identical twins look almost the same in these tags,8PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins but older twins show marked differences, changing which genes are active.9PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins As a group, identical twins also carry a lasting tag pattern from the moment the embryo split.10PubMed Central. Identical twins carry a persistent epigenetic signature of early genome programming So two twins with nearly the same DNA can still differ in health and traits over a lifetime. Standard DNA tests read letters, not tags, so tags don’t help tell twins apart in routine testing.
Your Own Cells Don’t All Carry the Same DNA
From fertilization on, cells pick up changes from copying errors and from damage like sunlight or smoking.11PubMed. Somatic mosaicism across human tissues Every person is a mosaic of cells with slightly different DNA.12PubMed. Somatic mosaicism and disease
You can’t see or feel any of this. Almost all of the small changes land in stretches that don’t affect how a cell works, so the cell carries on as normal. A change in one skin cell stays with that cell and its descendants, ending up in a small patch of tissue. The earlier in life a change arises, the more cells carry it, because it is copied into every cell descending from the first one. Routine forensic tests look at a small set of spots that rarely change, so your cheek swab and your blood still give the same standard profile.
Not every change is passed to children. Changes in skin, blood, or gut cells stay in the person, because children come only from egg or sperm cells. A change in the cells that make eggs or sperm can be passed on, and then the child carries it in every cell. That is how a child can have a change that neither parent shows on a test of their blood or cheek cells.
With age, mutations keep piling up, and most do little or nothing.13PubMed Central. Clonal hematopoiesis in human aging and disease But a change can give one blood stem cell an edge, so a large share of blood comes from that one cell.14PubMed Central. Clonal hematopoiesis in human aging and disease This takeover is very common in older people.15PubMed Central. Clonal hematopoiesis in human aging and disease It usually causes no symptoms and is found only by gene testing, but it is linked with a higher risk of some blood cancers and heart disease.
How Sure Is a DNA Match
Forensic labs compare a set of short repeating DNA stretches, usually 10 or more markers.16PubMed Central. Distinguishing genetically between the germlines of male monozygotic twins They then report the random match probability, which is the chance a random unrelated person from the population would match.17District of Columbia Department of Forensic Sciences. FBS22 – STR Statistical Calculations Guidelines The odds at each marker are multiplied together, so a full profile gives extremely small chances.18MetricGate. DNA Random Match Probability (RMP) Explained Calculator A figure like one in a trillion describes how often an unrelated person would match by chance. It is not the chance the suspect is innocent. It also assumes the person is unrelated to the source and the sample was clean, so it can be badly misleading if the true source is a brother or the sample is mixed. And courts and labs choose the reference group, so the number depends on which population the stranger is assumed to come from.
A close relative inherits from the same parents and shares many of the same markers, so the chance of matching at several markers is far higher than for a stranger. Full siblings can match at some markers and differ at others, so a full profile separates them even though a partial one may not.
Standard forensic tests cannot tell identical twins apart.19PubMed. Finding the needle in the haystack: differentiating “identical” twins in paternity testing and forensics by ultra-deep next generation sequencing But very deep sequencing has found five rare changes shared by one twin and his child but not the other twin.20PubMed. Finding the needle in the haystack: differentiating “identical” twins in paternity testing and forensics by ultra-deep next generation sequencing In one criminal case, the culprit among identical twins was identified this way.21PubMed. Identification of the perpetrator among identical twins using next-generation sequencing technology: A case report Once standard markers narrow it to the twins, a single differing change can be enough.22PubMed Central. Distinguishing genetically between the germlines of male monozygotic twins Labs sequence the whole genome looking for rare one-twin changes. It’s expensive and slow, so it is used mainly in serious cases or paternity disputes.
Messy samples are harder. Crime-scene samples often hold DNA from more than one person.23Forensic Science International: Genetics. The interpretation of low level DNA mixtures Tiny samples, under about 200 trillionths of a gram, can drop or blur markers.24PubMed Central. Current developments in forensic interpretation of mixed DNA samples
A match is also not the same as guilt. DNA shows whose cells are present, not how or when they got there. Touch DNA can transfer through handshakes or shared objects, so a match still has to be weighed with other evidence.
When One Person Has Two Sets of DNA
Two early embryos can fuse into one person, called a chimera, who carries two sets of DNA.25PubMed. Natural human chimeras: A review In one case, a father’s cheek, hair, semen and nail samples showed two DNA profiles while his blood showed one.26PubMed. Paternity pseudo-exclusion caused by tetragametic chimerism in a gestational surrogacy case His child at first appeared not to be his on a paternity test.27PubMed. Paternity pseudo-exclusion caused by tetragametic chimerism in a gestational surrogacy case His blood carried one profile while his reproductive cells carried the other, so the child matched the second profile that blood testing never showed. Retesting from a different tissue settled it.
Most chimeras never find out. Both sets of cells usually work together normally, so the person has no symptoms and looks like anyone else. Everyday tests such as a cheek swab or blood test sample only one tissue, which may carry only one of the two profiles. Discovery tends to come through a surprising paternity or maternity result, a test for another reason such as a transplant match, or sometimes patches of skin or two eye colours. Testing several tissues reveals the second profile.
Transplants add donor DNA. After a bone marrow transplant, much of the DNA floating in blood comes from the donor.28Clinical Chemistry. Predominant Hematopoietic Origin of Cell-free DNA in Plasma and Serum after Sex-mismatched Bone Marrow Transplantation Some non-blood cells in recipients can carry the donor’s DNA too.29PubMed Central. DNA chimerism and its consequences after allogeneic hematopoietic cell transplantation So a blood test, and sometimes even a cheek swab, after such a transplant may show someone else’s profile. Recipients should tell the testing lab or police about a bone marrow transplant. A cheek swab is usually more reliable than blood afterward, though it can still contain some donor DNA, and ancestry results from a blood sample may reflect the donor.
Can DNA Tell Close Relatives Apart
Mitochondrial DNA is a small loop of DNA in the cell’s energy structures, and it comes almost entirely from the mother’s egg. It passes almost unchanged from a mother to all her children.30Journal of Forensic Science and Research. Analyzing Maternal Inheritance of Mitochondrial DNA using PCR-RFLP A mother, her children, and her maternal relatives share nearly the same type. A match can confirm a maternal link but not pinpoint one person.
Y chromosome tests follow the father-to-son line. The Y passes largely intact, so brothers, their father, and paternal cousins carry nearly the same Y. These tests usually fail to separate men of the same paternal line,31PubMed. A new future of forensic Y-chromosome analysis: rapidly mutating Y-STRs for differentiating male relatives and paternal lineages so a standard Y test on one brother will match the other and can’t say which left a sample. Women don’t have a Y chromosome, so these tests only apply to men’s lines. Newer fast-changing Y markers can separate most close male relatives and nearly all distant ones.32PubMed Central. Large-scale pedigree analysis highlights rapidly mutating Y-chromosomal short tandem repeats for differentiating patrilineal relatives and predicting their degrees of consanguinity Standard Y kits even miss some unrelated men.33PubMed. Improving empirical evidence on differentiating closely related men with RM Y-STRs: A comprehensive pedigree study from Pakistan
Why Your Unique DNA Matters for Medicine
Gene editing aims to fix disease-causing DNA directly.34PubMed Central. Implications of human genetic variation on CRISPR-based therapeutic genome editing The editing tool is guided by a short stretch of letters that matches the DNA it should cut, a bit like a search for a specific address. If a person’s DNA has a different letter at that address, the tool may cut in the wrong place or fail to cut in the right one. Designers build the tool against a reference genome, a composite that doesn’t match every person.
A variant carried by some people created an unwanted cut site for one sickle cell editing treatment.35PubMed Central. Human genetic diversity alters off-target outcomes of therapeutic gene editing It made a second place that looked like the target, so the tool cut there too. Cells with the variant got stray edits in about 1 in 10 copies, while cells without it had none.36PubMed Central. Human genetic diversity alters off-target outcomes of therapeutic gene editing Cutting in an unintended spot can damage a gene the cell needs, which is a safety concern. The finding came from testing cells from people with and without the variant, which is why such testing is useful before treatment.
Donor DNA is useful in transplants too. Transplant teams track donor DNA in blood to spot organ rejection early.37PubMed Central. Donor-derived cell-free DNA as a diagnostic tool in transplantation Dying donor cells release DNA, and higher levels track with rejection.38PubMed Central. CELL-FREE DNA TO DETECT HEART ALLOGRAFT ACUTE REJECTION39PubMed Central. Cell-free DNA for the detection of kidney allograft rejection