Each year, twin brothers and sisters from across the planet travel to Twinsburg, Ohio, to participate in the Twins Days Festival, taking place this year on August 4-6. It is the largest gathering of twins in the world, and it will get even larger if current trends prevail.
From about 1915 to 1980, one out of every 50 babies born in the United States was a twin. That number has since skyrocketed to one out of 30, and there’s no indication it is abating. Although still rare, the dramatic increase in twin births can mean more negative health consequences for mothers of twins and her children, including premature births and low birth weight.
But for geneticists, twin births are something to celebrate. Twins provide a trove of biological information that scientists cannot get anywhere else. They’re valuable in helping scientists understand diseases and other conditions, including eating disorders, obesity, sexual orientation, and various psychological traits.
Twin studies can also give researchers new insights into how different lifestyles and habits affect two people with the same genetic blueprint. Studying twins is extremely useful in examining the effects of genetic and environmental factors that can influence inherited traits across generations.
For example, Scott and Mark Kelly were once identical twin brothers. Then Scott Kelly spent a year in Earth’s orbit aboard the International Space Station and all that changed. When he landed, he was two inches (5 cm) taller, had a lot less body mass, and according to NASA researchers, aspects of his DNA had changed. He and his brother were no longer identical.
Scott’s year in orbit changed his immune system, the way his bones formed, and his eyesight, along with other biological functions. Most of those genetic changes returned to normal. However, researchers found 7 percent of his gene expressions had also changed. Gene expressions determine whether genes turn off and on, which can change how cells function. Scientists were not alarmed because environmental factors, in this case the stress of space travel, can affect this process. Consequently, Scott’s body quieted some genes while amplifying others.
Though his gene expressions changed, his DNA did not. Moreover, that Scott and Mark are no longer “identical” didn’t come as a surprise. At the most basic sequence level, chemical changes can occur over time, affecting where and how genes are expressed, even in identical earthbound twins. In reality, Mark and Scott Kelly haven’t been “identical” for years. (Excerpt from ‘Twins can become ‘unidentical’—and more fascinating twin facts’, nationalgeographic.com, 2023)
According to the passage, how has the rate of twin births in the United States changed over the years?