The Genes of Twins – Is the tendency to have twins hereditary?

Toward the end of “Friends”, Monica and Chandler learn that they will be adopting twins. Once they had their baby, they were offered another child to adopt. They were surprised because it turns out Erica had been pregnant with two, while she was told she only had one. When Chandler heard this, he remarked “Interesting!”

Twinning is hereditary as long as it occurs from happening from fraternal twins. Fraternal twins starts with a fertilized egg, but if the embryo splits at the initial stages of cell death, then there might be two different babies who have the same genetic material.

Twins are categorized as either identical or fraternal. Identical twins occur rarely and happen when one fertilized egg splits into two embryos. Fraternal twins, on the other hand, are the result of two eggs being fertilized by separate sperm cells. These two types of twins have different characteristics and share only 4% of their genes.

The likelihood of having identical twins is not due to our genes, but rather the environment. In contrast, the formation of fraternal twins is mediated by the mother’s genes, which could be passed down in her heredity.

Learn why there are two sets of genes in people

A study, published in 2016, found natural markers on genes that correlate with the chance of having twins. These markers have been identified after testing the genetic code of moms who had fraternal twins. The chances of having fraternal twins are increased by 29% when a woman has 2 copies of these 2 mutations in her genes.

One specific gene that is important for egg maturation is the Follicle-Stimulating Hormone, which has a cycle like the one in a woman’s body. When levels of this hormone are high for a huge time period, then there is an increased chance that two eggs will mature simultaneously. This gene is what causes fraternal twins to happen.

A second gene found by the research is called SMAD3, and is related to the transmission of information within cells. If a cell is exposed to a hormone, this protein can modify the cell activity in accordance to the information from SMAD3. This product may affect how ovaries react to FSH, but its role in ovulation or pregnancy isn’t yet known.

Researchers are studying a gene that affects conception. They hope to understand how it works and if, in the future, they could use it to diagnose difficultly conceiving women and treat them with ART.

  New information on how twins can be created with assisted reproductive technology

With the use of ART, there has been a sharp rise in cases of twins. One of the possible treatments, In vitro fertilization, can lead to the birth of two or three babies at once. ART poses health risks for mothers and their children

Even without IVF, fertility treatments can result in the birth of twins. Some of these treatments include ovulation drugs, which slightly increase the probability of simultaneous maturation of multiple eggs. Other treatments include injections of FSH hormones, which, as previously described, are also liable to increase those probabilities. As a result, we see increasing numbers of cases where the mother’s genes don’t play a role in the birth of twins; they’re related only from the treatment she underwent to conceive.

What about Erica, the biological mother of the twins from the TV show Friends? Her pregnancy was supposedly unplanned and it is thus quite clear that she did not receive fertility treatments. She gave birth to fraternal twins – a boy and a girl – and considering her remark regarding the number of twins in her family, it is very likely that she carries one of the gene variants that lead to multiple births. Have you had any fertility treatment? Do you have any other twin pairs in your extended family?

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