It is true that identical twins share their DNA code with each other. This is because identical twins were formed from the exact same sperm and egg from their father and mother. (In contrast, fraternal twins are formed from two different sperm and two different eggs.)
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.
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.
Because identical twins have the same DNA, it is nearly impossible to distinguish between these individuals when analyzing DNA for paternity testing or for evidence of a crime.
The bottom line
Identical twins share a lot of similarities in both their genetic makeup and their physical appearances. But, like those who aren't twins, identical twins all have unique fingerprints.
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
We conclude that there is a greater perceptual resemblance between the voices of identical twins than between voices without genetic relationship. The identification however is not perfect. The voice pitch possibly contributes to the correct twin identifications.
Identical twins have 100% of their DNA the same while fraternal twins share only 50%. But what about semi-identical twins? Well, they have 75% of their DNA the same.
Like for their twin parents, there are many fascinating family relationships for the children of twins–when identical twins have children, their children are cousins but genetically as similar as half-siblings.
Blood or saliva could be used to sequence the twins' genomes. Researchers could look for genetic mutations that only one twin — the father — shared with the child. But the scientists' analysis also showed that such a test would have to be very precise and sensitive.
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).
Identical twins are the only siblings that share 100% of their DNA. Non-identical brothers and sisters share about 50% of inherited gene variants, which is why siblings and fraternal twins can be so different.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
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.
All human beings are 99.9 percent identical in their genetic makeup. Differences in the remaining 0.1 percent hold important clues about the causes of diseases.
Identical twins do not have identical fingerprints, even though their identical genes give them very similar patterns. 1 The fetus begins developing fingerprint patterns in the early weeks of pregnancy. Small differences in the womb environment conspire to give each twin different, but similar, fingerprints.
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
Identical twins live longer, possibly because they have a BFF from birth, according to researchers from the University of Washington, Seattle, WA. Furthermore, this effect is greatest in male identical twins, according to results from their recent study, published in the journal PLOS ONE.
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.
According to The American Society for Reproductive Medicine, women who are fraternal twins have a 1 in 60 chance of having twins, and men who are fraternal twins have a 1 in 125 chance of fathering twins. It was previously believed that identical (monozygotic) twins were random — not genetic.
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.
Because you and your sister are identical twins, you pretty much have the same DNA.
In twin studies looking at schizophrenia, scientists have shown that if one identical twin has schizophrenia, then there is a 30-50% chance that the other twin will have it as well1. If a fraternal twin has schizophrenia, then there is a ~15% chance that the other twin will have it too1.
Is there a third type? Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called polar body or half-identical twins.
The term mirror twin is used to describe a type of identical, or monozygotic, twin pairing in which the twins are matched as if they're looking into a mirror — with defining characteristics like birthmarks, dominant hands, or other features on opposite sides.