Many Identical Twins Actually Have Slightly Different DNA (2024)
Research published on January 7 in the journal Nature Geneticsshows that identical twins differ by an average of 5.2 genetic mutations. The authors argue that these small differences between twins’ genetic code could change how scientists study human development.
The study of 381 pairs of identical twins and two sets of identical triplets found that only 38 were genetically identical, Tina Hesman Saey reports for Science News. Most had just a few points of genetic mismatch, but 39 had more than 100 differences in their DNA. The findings could impact future studies of the ways that the environment affects disease and human development. In such studies, scientists often assume that pairs of identical twins have identical DNA, so their differences can be explained by the environments they grew up in.
“Before you can make that interpretation, you’d better make sure that one of them does not have a de novo [randomly introduced] mutation in an important gene that the other one does not,” says Kari Stefansson, CEO of DeCODE Genetics and lead author of the new study, to the Scientist magazine’s Catherine Offord. “So this certainly places a new kind of burden on those who use identical twins to establish the separation between nature and nurture.”
The researchers sequenced the full DNA code of cells found in cheek swabs and blood samples from not only the twins, but also their parents, their children, and their children’s other parent, Nicoletta Lanese reports for Live Science. With three generations of genetic information, the researchers could not only figure out where specific mutations appeared in the DNA code, but when, by determining at what stage in their embryonic development the mutations occurred.
The genetic mutations observed in the study mostly involved a single building block of the DNA code, called a base, mistakenly swapped for a different one. In some cases, letters were either inserted or deleted. Working at the scale of a human genome, a few changes may not cause much of a difference.
"Such genomic differences between identical twins are still very rare,” says University of Pennsylvania computational biologist Ziyue Gao, who wasn’t involved in the study, to Live Science. The human genome is six billion base pairs long, and the study did not make it clear how many of the mutations would cause significant changes. She adds, “I doubt these differences will have appreciable contribution to phenotypic [or observable] differences in twin studies.”
The mutations covered in the study tend to appear while a cell is replicating, or dividing itself, to create more cells. During that process, the cell has to completely copy all six billion base pairs, and sometimes it makes mistakes. The cell has machinery to correct mistakes, but even those go wrong sometimes, leaving behind small typos. If such a typo is created in a bundle of cells early in human development, then when the bundle splits into two parts, the mutation might not reach both parts equally.
“We have found a twin pair where one of the twins has mutations in all cells of his body, and they are not found in any cell in the body of the other twin. That means basically that one of the twins is formed solely from the descendant of the cell where the mutation took place,” says Stefansson to the Scientist. “Then we have found twins when the mutation is found in all cells in the body of one of the twins, and in 20 percent of the cells in the body of the other twin. So one of the twins has just formed from the descendant of this one cell where the mutation happened, and the other is formed in part by descendants of that cells and in part by something else.”
The study compared the twins’ DNA to their parents and children to find out when the mutation happened during their development. If the mutation happened very early, then the mutation could be passed along to their children, because it would have ended up in their sperm or eggs. Mutations that happen later in embryonic development might only appear in a person’s non-reproductive cells.
Uppsala University geneticist Jan Dumanski tells the Associated Press’ Christina Larson that the study is “a clear and important contribution” to medical research.
Stefansson adds that the implications go beyond nature-and-nurture twin studies.
“This is not just a study that has relevance when it comes to understanding of the genetics, but also human development: How do we probe early human development in an ethical manner, a non-interventional manner? This is one way of doing that,” he says to the Scientist magazine. “We can begin to use the mutations to develop [an] understanding of how cells are allocated from the early embryo to develop the various organs in the body.”
They are identical because both siblings have the same set of DNA from mom and the same set of DNA from dad. Since both sets are the same they share 100% of their DNA. Fraternal twins are really just siblings who share the same womb. Each starts out as a separate egg fertilized by a separate sperm.
Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes.
Some identical twins can have different heights and weights. This is because height and weight are controlled by what you eat as well your DNA. Differences in diet can start earlier than you might imagine. When the twins are growing inside the uterus, there can be differences in how well they connect to the placenta.
Even though identical twins do share a significant amount of similar DNA, there are clear genetic mutations between each member, proving that they're not carbon copies of each other.
Cantor explains that in most instances, a pair of identical twins share the same DNA when they split. However, she continues, a recent report found that some developing twin embryos may already have genetic differences.
These twins are very much alike but aren't a 100% DNA match — sharing about 75% of their genetic DNA. (Less than identical twins but more than fraternal twins.) To date, polar body twinning remains a theory.
These twins can be the same or different sexes and are no more alike than any brother or sister, despite being born together. In this case of semi-identical, or sesquizygotic, twins, the egg is thought to have been fertilised simultaneously by two sperm before it divided.
Identical twins are born when one zygote (formed by a sperm and an egg cell) splits into two foetuses. That's why these rare cases are known as monozygotic twins: the two children are formed by the same gametes, inherit the same genes, and therefore have identical DNA test results.
As monozygotic (identical) twins develop from a single egg fertilized by a single sperm, which splits after the egg starts to develop, they are expected to share all of their genes, whereas dizygotic (fraternal) twins share only about 50% of them, which is the same as nontwin siblings.
1. Elvis had a twin. On January 8, 1935, Elvis Aron (later spelled Aaron) Presley was born at his parents' two-room house in East Tupelo, Mississippi, about 35 minutes after his identical twin brother, Jesse Garon, who was stillborn.
That's because so-called identical twins pick up genetic mutations in the womb, as their cells weave new strands of DNA and then split into more and more cells. On average, pairs of twins have genomes that differ by an average of 5.2 mutations that occur early in development, according to a new study.
These types of twins share a chorion, placenta, and an amniotic sac. This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.
107-year-old Japanese sisters confirmed as world's oldest identical twins. Umeno Sumiyama and Koume Kodama (Japan) have been confirmed as the oldest identical twins living (female) and the oldest identical twins ever (female) at 107 years and 300 days old as of 1 September 2021.
Because of recombination, siblings only share about 50 percent of the same DNA, on average, Dennis says. So while biological siblings have the same family tree, their genetic code might be different in at least one of the areas looked at in a given test. That's true even for fraternal twins.
On average full siblings will share about 50% of their DNA, while half siblings will share about 25% of their DNA. The actual amount may vary slightly since recombination will shuffle the DNA differently for each child.
Occasionally, two sperm are known to fertilize a single egg; this 'double fertilization' is thought to happen in about 1% of human conceptions. An embryo created this way doesn't usually survive, but a few cases are known to have made it — these children are chimaeras of cells with X and Y chromosomes.
Polar body twinning is thought to occur when an egg splits - and each half is fertilised by a different sperm. This results in twins who appear very much alike but share approximately 75% of their DNA.
A parasitic twin is a type of conjoined twin where one fetus stops developing but remains attached to its twin. The other twin continues to develop, but is usually born with the limbs, organs or other tissue structures from its parasitic twin still attached. It's a very rare condition.
On average, we are just as related to our parents as we are to our siblings--but there can be some slight differences! We share 1/2 of our genetic material with our mother and 1/2 with our father. We also share 1/2 of our DNA, on average, with our brothers and sisters. Identical twins are an exception to this rule.
On average, over the 22 pairs that aren't XY, full siblings will share around half the DNA on each chromosome pair. Half siblings will share half their DNA on only one of each pair. Half of half is 25%.
Identical twins have DNA that is 99.9% the same. They also have almost nearly identical brain wave patterns. But they do have different fingerprints and teeth marks! 22% of twins are left-handed as opposed to the 10% of singletons who turn out to be left-handed.
Half siblings share 25 percent of their DNA. 50 percent of each half sibling's DNA comes from the shared parent, and they inherited about half of the same DNA from that parent as one another.
Identical twins will always have the same blood type because they were created from the same fertilized egg (fraternal twins can have different blood types — again, providing the parents do — because they are created by two fertilized eggs).
Elvis looked over at Vernon and said, 'As a matter of fact, Daddy, from now on, I want my name written with two 'A's, espesially on legal documents'. That's why it has two 'A's on the gravestone. (Below, Vernon Presley at Elvis' grave at Graceland.)
Amazingly Elvis was actually a natural blonde until his late teens and even after when his hair began to go darker it wasn't naturally the shade we all know so well. It was usually dyed a shade of brown known as 'Mink Brown', but once when the King himself tried to do it, he opted to use black shoe polish.
A 100% DNA match on Ancestry means that your relationship to your relative is almost certainly one of the relationships listed in the relationship column. We will typically only have 100% probability DNA matches for relatives who are either descended from us, or very closely related.
Because of recombination, siblings only share about 50 percent of the same DNA, on average, Dennis says. So while biological siblings have the same family tree, their genetic code might be different in at least one of the areas looked at in a given test. That's true even for fraternal twins.
Can a half-sibling show up as a cousin? While the amount of DNA you share is different between half-siblings and cousins, your half-sibling still may show up as a “first cousin” because your centimorgans may be within the 1,300 range.
Yes, it is possible to share a small amount of DNA with someone and not be related. In other words, it's possible to share genetic material and not share a common ancestor or any identifiable genealogical connection.
On average, over the 22 pairs that aren't XY, full siblings will share around half the DNA on each chromosome pair. Half siblings will share half their DNA on only one of each pair. Half of half is 25%.
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