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Researchers have long known that women whose families include fraternal twins are more likely to give birth to twins themselves, and they’re finally starting to figure out why. After scanning data from nearly 2000 mothers of fraternal twins, scientists from eight countries found two genes that increase a woman’s chance of having twins—one that affects hormone levels and another that may alter how ovaries respond to them. The second of these may also have implications for why some women respond better than others to in vitro fertilization.
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 to compare identical and fraternal twins to understand how much variation in a trait is due to environment versus genetics. Because of that, several large databases track twins as they age. In 1987, a young behavioral geneticist at Vrije Universiteit in Amsterdam named Dorret Boomsma started the Netherlands Twin Register, which now contains more than 75,000 twins, triplets, and other children of multiple births.
Researchers like Boomsma have some ideas, especially as fraternal twin births are on the rise in Western countries—for instance, the United States saw a 76% increase from 1980 to 2011. In vitro fertilization, for which demand has surged, is more likely to yield twins. Older women, who are having more children than in the past, are also more likely to release more than one egg, increasing their chances of giving birth to fraternal twins.
Read full, original post: Having fraternal twins is in your genes — and in your hormones