"[Two sets of] identical twins will not have identical offspring," says Professor Rodney Scott, head of medical genetics at the University of Newcastle. The only way we get identical offspring is when a fertilised egg splits into two cells. "Those identical cells grow into identical twins," says Scott.
It was once believed that identical (monozygotic or MZ) twins occured at random. There is now some evidence to suggest that MZ twins may run in families, but this is very rare.
Here are the odds of having twins:
1 in 85 in general. 1 in 250 has identical twins. 1 in 17 if the mother is a fraternal twin. 1 in 85 if the mother is an identical twin.
Since identical twins share the same DNA, the children of two pairs of identical twins are legally cousins, but genetically more similar to siblings.
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. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
If a set of identical twin brothers had children with a set of identical twin sisters, all of their children would be genetically similar to being full siblings instead of cousins. If they're all fraternal twins the children would be double cousins, but not any more similar than if the parents were regular siblings.
Despite the name, identical twins are rarely completely identical. Their skin tone, weight, height, or personality, to name a few characteristics, may be different. How does this happen? Every time a cell divides—as occurs in the developing embryo—your DNA must replicate, a very complicated process.
And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.
They come from the same fertilized egg and share the same genetic blueprint. To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.
If one identical twin has autism spectrum disorder (ASD), the other twin has a 76 percent chance of also being diagnosed with it. The numbers are lower for fraternal twins. The percentage of fraternal twins who each share an ASD diagnosis is 34 percent for same-sex twins and 18 percent for boy-girl pairs, Dr.
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. It sounds like fraternal twins do indeed run in your family!
The quick answer to this question is that, in a twin pregnancy, it is the mother's genes that determine twins. First up, giving birth to identical twins is not genetic, but conceiving fraternal twins is. The mother may have the genetic trait of releasing two eggs in one menstrual cycle.
There is no evidence of twin telepathy. There are many myths about twins—hence, the title of my new book, Twin Mythconceptions: False Beliefs, Fables, And Facts About Twins (2017, Elsevier).
In the mother's womb (uterus), most identical twins share the same placenta. (They get oxygen and nutrients from the mother and get rid of wastes through the placenta.) But they usually grow within separate amniotic sacs. In rare cases, identical twins share one amniotic sac.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA. A boy has XY chromosomes and a girl has XX chromosomes. Girl-boy twins occur when one X egg is fertilized with an X sperm, and a Y sperm fertilizes the other X egg.
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.
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
Monozygotic (identical) twins will have the same blood type, with a few very rare exceptions. Dizygotic (fraternal) twins may have the same blood type, or they may have different types. Therefore, it may be concluded that twins with differing blood types are dizygotic, or fraternal.
If more than one sperm fuses—a condition called polyspermy—multipolar or extra mitotic spindles are formed, resulting in faulty segregation of chromosomes during cell division; nondiploid cells are produced, and development usually stops.
As identical twins get older they may look more and more different, because they are exposed to more diverse environments. The science of epigenetics explains how these environmental influences can affect the genes.
Can twins have different fathers? In rare cases, fraternal twins can be born from two different fathers in a phenomenon called heteropaternal superfecundation. Although uncommon, rare cases have been documented where a woman is pregnant by two different men at the same time.
Twin sisters who married twins reveal their sons are genetic brothers and cousins. Identical twin sisters who married identical twin brothers have given birth to sons who are brothers and cousins at the same time.
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.
In identical twins, if a twin develops schizophrenia, the other twin has a 1 in 2 chance of developing it, too. This is true even if they're raised separately.