Maybe you like warm weather and hate the cold, or maybe you feel great during a brisk winter day. Scientists think that whether you’ll be happier in Florida or Vermont is encoded in your genes. And if you’re living in the wrong climate for your genetic makeup, it can make you get fat.

Geneticist Douglas Wallace found that these genes are in the cells’ mitochondria, which is passed down through the female line. Researchers think these were once independent bacteria that evolved to become part of the human cell, because they contain a separate string of DNA coding. Mitochondria are important when it comes to taking in and burning calories.

Wallace found that variations in this DNA seem to correspond to the place where you live and that there are especially big differences between people living in warm and cold climates. The greatest variety in mitochondrial genetic codes is in Africa, where humans started out. Several variations, all branching from two lines in Africa, have been found in Europe and Asia. But in northern regions such as Siberia, they?ve found only a small number of gene variations. “Why does a founding population have a tremendous amount of variation, but only two small components of that variation give rise to Europe variation?” Wallace says. “And if European variation is great, why do you only find two kinds in the extreme Arctic?”

Efficient mitochondria work like a steam engine, producing a lot of energy without losing too much heat. Less efficient mitochondria give off heat while producing energy. In cold climates, people evolved to have inefficient mitochondria so they would release more heat and keep warm. In places where it’s already warm, excess heat isn’t needed, so most calories can be used directly for work. “This is why people from colder regions can often eat more,? Wallace says. ?Their systems are made to give off more heat so you have to eat more calories to survive.”

Wallace has been testing this idea is by measuring the swimming speed of men’s sperm. Mitochondria power the tail of each sperm so that it can swim to the female egg. His colleague, Eduardo Ruiz-Pesini, found a way to have a “race” between the sperm of men from warm and cold climates. He found that men from warmer regions with genes programmed for more efficiency had faster swimming sperm than men from colder regions.

This could also be one reason why Americans struggle with obesity. When people with efficient genes move to colder climates, their mitochondrial DNA doesn’t change, but their diets do. Their mitochondria continue to get a lot of energy from just a few calories, while they?re taking in more calories, trying to keep warm. This leads to weight gain. The opposite is true if someone with ancestors from a cold climate moves to a warmer place. They often feel uncomfortable since their mitochondria are programmed to produce extra heat. “Not a lot of people from temperate zones like to live in the tropical regions,” says Wallace. “The first thing they do is install air conditioners.”

In the U.S., we have a lot of fat folks and a lot of air conditioners, so maybe we need to find out where our genes want us to go?and move there.

People who live in different places burn calories differently, and they dream differently too!

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