Why Do Fraternal Twins Run in Families?


Why do fraternal twins (twins who develop in two separate eggs) seem to run in families? It’s all about genes. A team of researchers say they’ve identified two genes associated with giving birth to twins, and their results are published in the American Journal of Human Genetics. 

Twin births are on the rise in America. In 1980, one in every 53 babies was a twin. By 2009, that number was one in 30. In almost every state (and D.C.), the rate of twin births has increased by more than 50 percent in the last three decades.

What’s causing it? It’s complicated. Fertility treatments, which often result in multiple births, are definitely on the rise. But parenthood is changing in other ways. Fraternal, or dizygotic (DZ) twin births are more likely in women with a higher body mass index (BMI), as well as mothers over the age of 40—two demographics that have increased since the 1980s. And then there’s another element: Many of these DZ twins are born to women who are related to one another. 

Researchers therefore believed there must be a genetic component to twinning, yet previous searches for a DZ-twinning gene had been unsuccessful. So rather than look for a twinning gene specifically, an international team of scientists combed through the genomes of women both with and without DZ twins, looking for commonalities. 

They began by compiling data from genetic databases in three countries: the Netherlands Twin Register and the Netherlands Study of Depression and Anxiety; the Australian Twin Registry; and a study from the Minnesota Center for Twin and Family Research. All told, the researchers were working with genetic information from 1980 mothers of DZ twins and 12,953 more control subjects. They conducted a sweep of all the subjects’ genomes, which yielded three gene variants that seemed more common in women who had DZ twins. 

The team then sent that data to researchers in Iceland. Analysts there scanned the genomes of another 3597 mothers with twins and 297,348 controls, looking for the variants the original team had identified. 

Two of the three variants reappeared with some frequency in the Icelandic group of mothers of DZ twins. One variant is associated with follicle-stimulating hormone (FSH), a hormone that causes growth in a woman’s ovaries. Women with this variant tend to make more FSH than others. When ovaries grow quickly, they release eggs more quickly, which could lead to the release of multiple eggs at one time. And if multiple eggs are fertilized, they could lead to multiple babies. Women with this variant were 18 percent more likely than others to have twins. 

The other variant lives in a gene called SMAD3, which influences how cells communicate with one another. The researchers think this variant may increase a woman’s sensitivity to FSH, so that even a normal amount of FSH could trigger the release of multiple eggs. Having the SMAD3 variant raised a woman’s relative risk of twins by 9 percent. Having both variants pushed the odds to 29 percent. (The researchers called it a "risk" because, as they note, having twins can be risky for the health of mothers and babies.)

Image Credit: Hamdi Mbarek

These two variants are probably just two of many, the researchers say. "There is a very clear suggestion and indication that more loci are contributing to the risk of having dizygotic twins as well," Dorret Boomsma, a biological psychologist at Vrije Universiteit (VU) Amsterdam, said in a press statement. "We've characterized the first two that have been replicated."

Banana Panic! Is the Fruit Really on the Brink of Extinction?

Enjoy your banana pancakes while they last. A tropical fungus that causes Panama disease could wipe out the Cavendish—the most popular variety of banana—in the not-so-distant future. To make matters worse, a wild crop of bananas in Madagascar that could help diversify the fruit's gene pool and protect the Cavendish was just put on the extinction list, the BBC reports.

Throughout the world, the Cavendish is the type of banana that’s most commonly consumed. The Madagascan banana produces seeds, rendering it inedible, but researchers at the Kew Madagascar Conservation Centre say it could be cross-bred to create a new kind of banana that’s both tasty and resilient. In other words, the key to saving the fruit is expanding its genes. Although it is nearly extinct, the African variety seems particularly promising because it could have unique properties that make it resistant to drought or disease.

"It doesn't have Panama disease in it, so perhaps it has genetic traits against the disease," Richard Allen, senior conservation assessor at the Royal Botanic Gardens, Kew, told the BBC. "We don't know until we actually do research on the banana itself, but we can't do the research until it's saved."

Banana panic has been ongoing for a few years, but Snopes reports that some fears—like claims that bananas will be extinct within a decade—are unfounded. However, threats to the Cavendish banana are very real, and it’s not the only crop that’s vulnerable. The world’s most popular wine grapes are also susceptible to pandemics for similar reasons, including a lack of genetic diversity. Scientists are now crossbreeding different varieties of grapes—much like efforts to save the banana—in an effort to keep the wine flowing.

[h/t BBC]

Why the World's Most Popular Wine Grapes Are Vulnerable to a Pandemic

When you're in the wine shop looking for the right wine to pair with your meal or bring to the party, the variety on the shelves seems rich and diverse, their taste influenced by the grape, soil, climate, and age. Among the most famous are the French "noble wines"—cabernet sauvignon, merlot, pinot noir, chardonnay, riesling, and sauvignon blanc—so called for being associated with high quality and easy growth in a variety of places.

But it turns out that many of the most famous grapes in the world are like nobility in another way: They're as inbred as a royal family, and have been for hundreds—and in some cases thousands—of years.

"Scientists are getting really concerned that this is setting up the perfect scenario for a great pandemic," Kevin Begos, whose new book, Tasting the Past, explores the history, archaeology, genetics, and future of wine, said at a recent book release event in New York City. They fear that a single merciless pathogen could wipe out many grapes around the world in the same way that a single fungus, Phytophthora infestans, eradicated the variety of potato common across Ireland in the 1840s, causing the great famine.

The vast majority of wine produced across the world derives from a single grapevine species: Vitis venifera. The domesticated grape has thousands of varieties, and quite a lot of genetic diversity among them, according to a 2010 paper in PNAS that analyzed genome-wide genetic variation of more than 1000 samples of V. vinifera subsp. vinifera and its wild relative, V. vinifera subsp. sylvestris. But that's not true for all grapes: Nearly 75 percent of cultivars had a first-degree relationship to at least one other. They were either parents or children.

The most popular commercial wines are made from a handful of these inbred grapes. Sauvignon blanc, for instance, has a first-degree relationship with cabernet sauvignon, cabernet franc, and chenin blanc, among many others. That genetically cozy family isn't unusual. You see it all over the grapevine.

Another problem is how grapes reproduce in vineyards. Instead of pollinating these hermaphroditic plants or growing them from seeds, as might happen naturally, grape growers generally make new plants from cuttings of existing ones, essentially cloning the same vines over and over.

They use this method to produce consistent flavor quality—and it's nice to decant a bottle of your favorite wine and know what to expect with the first sip. But this practice has kept some popular grapes in relative genetic stasis for a long time. Take pinot, parent of chardonnay and gamay, which has been cloned for 2000 years. Genetically, it's remained virtually unchanged—but the organisms that prey on it have not. "All those insects and pathogens and mildews that attack grape vines have been evolving," Begos said. "And they always figure out new ways to attack the grape vines."

Despite the wide use of pesticides—in the last 10 years, 260 million pounds of pesticides were put on wine grapes in California alone—"the industry is losing the arms race to the pathogens," Sean Myles, an author of the 2010 PNAS grape genome study, told Begos in Tasting the Past. "It’s really only a matter of time. If we just keep using the same genetic material, we’re doomed.”

The good news is that grape diversity could be the key to preventing rosé season from disappearing. Scientists are looking outside the noble wines and their popular cousins to old, wild, and lesser-known varieties, which "turn out to have natural disease resistance, and they've kept evolving," Begos said.

The idea is create hybrids selected for specific traits—not just pest resistance, but an ability to withstand greater heat in an era of climate change, adaptability to a wider variety of soils, and other resilient qualities.

One effort is VitisGen, a USDA-funded project involving researchers from a handful of American universities, including UC Davis, Cornell University, and the University of Minnesota. By studying the genomes of a variety of grapes, they're creating an enormous database of genetic traits. They're also experimenting with crossbreeding. Some of this genetic tweaking is decidedly old school, including pollinating grapes by hand.

Begos tells Mental Floss that they're especially interested in developing grapes that are resistant to downy mildew (Plasmopara viticola), a potential plague a la the potato famine. It can cause total crop loss if not controlled.

When it comes to selecting traits, it probably won't be flavor they'll be pulling from wild grapes, which "are really kind of terrible," Begos said. (In Tasting the Past, he quotes wine experts who describe the flavor of a fox grape as combining "animal fur and candied fruits.”) It's generally hardiness they're looking for. The concord grape in your kid's PB&J, for example, is "really tough," Begos said. Select some of its hardy genes and cross them with, say, the peppery flavor genes of the syrah grape—which the researchers have also identified—and maybe you can create a genetically resilient hybrid.

"The University of Minnesota has already had success identifying cold-hardy wine grape genes, and breeding them into new varieties that have impressed the toughest critics," Begos says, pointing to a 2015 top 10 wine list from New York Times food critic Eric Asimov. Number two on the list was made from hybrid grapes developed by UM.

You can do your part to encourage wine diversity by getting adventurous with your vino, trying a grape you've never heard of or blends from new regions. Check out organic and small wineries, which are experimenting with old cultivars and new varieties. And don't be afraid of a future with genetically tweaked grapes. We've been modifying them as long as we've been growing them. As Begos writes of these efforts, "At heart they’re unlocking flavor, disease-resistance, and growth genes that may be tens of millions of years old. To me these scientists are doing exactly what ancient Babylonians, Egyptians, and Greeks did: refining wine grapes to produce tastes we enjoy."


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