How different can identical twins be

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Get involved. Zygosity testing. Frequently asked questions. Update your contact details. Researchers 8. In a very small number of identical twins, splitting might happen even later.

In this case, both twins share an inner sac , called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins.

For example, a twin who receives less blood from a shared placenta might weigh less at birth. Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins. Women who are pregnant with twins sharing a placenta need to be checked more often than women with twins with separate placentas.

Frequent checks can pick up early on any potential complications. Twins sharing an inner sac monoamniotic are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply.

These twins are checked even more closely. Medical professionals often recommend that these twins are born earlier than other types of twins. Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, the placentas should be looked at to confirm or determine what type of twins they are.

Same-sex twins with separate placentas can be fraternal or identical. To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. The case of Gemma and Leanne Houghton sure seems paranormal. The two twins were teenagers and Gemma suddenly felt compelled to check on her sister. But as the deputy editor for Skeptical Inquirer magazine, Benjamin Radford, pointed out in an article on twins, Leanne had a history of seizures and her sister had been told to keep an eye on her.

No telepathy needed. But the most persistent myth about so-called identical twins is baked into the name. Monozygotic twins are not identical, and this realization has implications for scientific research. We expect monozygotic twins to have identical DNA. Given that our genome consists of 3 billion letters, comparing the DNA of monozygotic twins to see if it really is identical used to be impossible.

Shortcuts had to be employed, like testing for specific regions of the DNA known to vary a lot. However, improvements in technology and in its affordability now allow scientists to take a much deeper look, and they are finding out that many monozygotic twins are not identical at the DNA level after all. When scientists recently read the DNA of twin pairs , they reported that, on average, so-called identical twin pairs differed by 5.

This is tiny on the scale of 3 billion letters but it is an average. Thirty-nine of these pairs actually differed by more than mutations, while 38 pairs did not differ at all at the level of their DNA. Where do these mutations come from? Simply put, many of them arise because the enzyme tasked with copying our DNA makes mistakes, and if these mistakes go uncorrected, they stick around as mutations.

The more our DNA gets copied, the more opportunities there are for mistakes. When a mutation arises after the cell mass has split itself into two twins, one twin gets the mutation while the other does not.

But even if the mutation arises before the twinning process, it may affect only the cells that will make up one twin and not the other, or it can affect some of the cells of one twin, meaning that the adult twin will have the mutation in some of their cells but not in others, a phenomenon known as mosaicism.