Researchers have long known that women whose families include fraternal twins are more likely to give birth to twins themselves -Same, and they are finally beginning to understand why. After scanning the data of nearly 00 mothers of fraternal twins, eight countries scientists have found two genes that increase the chances of having twins and one that affects hormone levels and another that can change how the ovaries react to them in a woman. The second of these may also have implications for why some women respond better than others to IVF.
Unlike identical twins, who are genetically the same, fraternal twins are no more closely related in terms of DNA than regular siblings. But scientists often like comparing identical and fraternal twins to understand how the variation of a trait is due to the environment versus genetics. Because of this, several large databases follow the twins as they age. In 1987, a young behavior geneticist at Vrije Universiteit in Amsterdam named Dorret Boomsma began the Netherlands Twin Register, which now contains more than 75,000 twins, triplets, and other children of multiple births. Parents of all participants had the same question: Why did they twins? "People want to understand," said Boomsma.
Researchers like Boomsma have ideas, especially since the birth of twins are on the rise in Western countries, for example, the United States saw an increase of 76% from 1980 to 2011. in vitro fertilization, for which demand has surged, is more likely to give twins. Older women who have more children than in the past, are also more likely to release more than one egg, which increases their chances of giving birth to fraternal twins.
To permanently solve the mystery, a new team led by Hamdi Mbarek, a molecular geneticist at Vrije Universiteit, combined data from nearly 00 mothers fraternal twins from databases in the Netherlands, the Australia, and Minnesota, and compared them against the women who had not had twins, and women who had been identical twins. The researchers looked for individual bases of DNA called single nucleotide polymorphisms, or SNPs, which may vary from person to person, especially those that popped up most often in mothers of fraternal twins and not in others . Once they came up with a few potential SNPs, they ran the analysis once more in a separate database of Iceland, reducing their key findings down to only two SNPS. A copy of each SNP increased the odds of having fraternal twins a mother of 29%, they report today in The American Journal of Human Genetics .
The first SNP is near a gene called FSHB , which is involved in the production of follicle-stimulating hormone ( FSH ). FSH levels fluctuate as the eggs mature in the ovaries, if levels remain too high for too long, the ovaries release multiple eggs, the first of a series of events that could lead to a twin birth . It is therefore not surprising that FSH gene is associated with having twins.
The second SNP was more of a surprise. He was in a gene called SMAD3 . By changing the way the molecules reported to each other, SMAD3 changes how the ovaries react to FSH , at least in mice. Although the role SMAD3 s' is somewhat of a mystery here, Mbarek said SMAD3 could be a candidate to understand why some women respond better to IVF others . "This is a new area of biology that was found in this document," said Anna Murray, a geneticist at the University of Exeter in the UK, who was not involved in the study. She agrees this is an interesting avenue to explore, but stressed that the study is only a first step.
Mbarek next plans to investigate whether women with the variant SMAD3 are more likely to get pregnant with IVF. But for now, it's exciting to know anything at all about genetics. "This is the first demonstration of the robust genes that are involved in [having fraternal twins]," Murray said. For his part, Boomsma is delighted to have seen the story through its full 30-years of the first entry in the Dutch register today's conclusions.
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