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Why Do Fraternal Twins Run in Families?

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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."

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Scientists Figure Out Why Roses Don't Smell as Good as They Used To
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Roses are red, violets are blue, but they just don't smell like they used to.

A team of 40 international researchers has successfully mapped an heirloom rose's genome and learned where the bud's color and scent come from—and how to tweak those traits to yield a more fragrant flower. Historically, rose breeders have opted for pretty petals over pleasant perfumes, and as a result, the rose's natural scent has faded over time, according to Science News.

The study, published in the journal Nature Genetics, reports that some of the genes of the "Old Blush" pink China rose cancel each other out, "with some turning on to brew a scent component while others shut down manufacture of anthocyanin pigments needed for rosy petals," Science News reports. The researchers also found 22 new biochemical steps in the production of terpenes, the volatile organic compounds key to the rose's perfume. With a better understanding of the complex relationship between color and scent, breeders of both roses and other plants could start producing flowers without sacrificing one trait for the other.

"The big challenge is you need to know what to edit," Todd Mockler, a plant researcher who was not involved with the rose study, tells The New York Times. “You can't just randomly start editing. You have to know what to target. The only way to know that is to have a genome sequence.”

The rose is most closely related to the strawberry plant, but it also has family ties with the apple and pear. Given that modern roses contain a blend of genes from between eight and 20 different species, mapping its genome was no small feat. It took researchers eight years to complete this study, according to the BBC. And while it's not the first time the rose genome has been mapped, this new analysis is far more comprehensive.

Similarly, the sunflower contains a complex genetic code, but scientists were able to map its genome last year, serving to aid future researchers and flower breeders. 

[h/t BBC]

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Big Questions
How Do Eyes Get Their Color?
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Paul Newman wasn't too fond of his blue eyes. The actor, who earned admiration from audiences and critics in everything from Cool Hand Luke to The Verdict, had a piercing set of blue irises that were as recognizable as Sylvester Stallone’s deltoids. He found the attention they received slightly grating. “If blue eyes are what it’s all about … I may as well turn in my union card right now and go into gardening,” the actor/philanthropist told The Saturday Evening Post in 1968.

If Newman knew the science behind his distinctive peepers, maybe he wouldn’t have been so hard on them. Although your genes are responsible for the color of your eyes, it’s a very complicated hereditary trait. Where you fall on the spectrum from light Newman blue to dark brown is the result of how much melanin pigmentation you have.

The iris—the colored part of the eye surrounding the pupil—is made of layers. The iris pigment epithelium is in the back and has some black or brown pigmentation to it. The layer over it is the stroma, which sometimes has brown melanin pigment, as well as colorless collagen. The black dots or “strings” you see in the eye are typically coming from the epithelium and are visible through the stroma.

Color is determined by the amount of melanin in the stroma. If you have brown eyes, you have brown melanin in the stroma that will absorb available light and make the iris appear darker in color. If you have green eyes, there’s not much melanin or collagen, and the light will be scattered. If your eyes are blue, you have no melanin at all—all of the light hitting the eye is scattered and reflected back. That’s why people with blue or green eyes can seem to shift eye color, depending on the amount of light in a room.

So how are genes involved? While they don’t directly program your body for a certain eye color, they do affect the quality and quantity of melanin in the stroma, which dictates your hue. While Newman’s brand of blue is a little more unusual than brown—the most common color—he probably would’ve been equally perturbed by grey. That shade, possibly caused by excess collagen, is considered the rarest eye color of all.

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