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Herding Hemingway's Cats:
Understanding How Our Genes Work
(Some cats are polydactyl - they have six toes. Hemingway was fond of them, and a colony of them still live on his estate today.)
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Some henes need to be turned on in all our cells. These are 'housekeeping' genes, and they do the day-to-day jobs of building cells, providing the energy to run them, taking out trash etc. But genes which produce the proteins that produce individual characteristics - to be bones or skin or muscle in different parts of body, should only turn on when and where they're needed as a baby grows in the womb.
Contrary to what most people think, a large part of evo has little to do with changing the genes. It has to do with how the genes are regulated.
Genes determine proteins get encoded. Between the genes are genetic switches, which determine whether the gene is on or off. And that is determined by which proteins (called transcription factors) bind to them at any given time.
Three-spined sticklebacks are small fish that normally live in the ocean but which go back to coastal rivers each spring to breed. But about 12,000 years ago, at the end of the last ice age, groups of sticklebacks got trapped in fresh water lakes. Since then, each isolated population has developed separately, and there are now 30 different species, each strikingly different to their sea-dwelling cousins.
Sea-going sticklebacks have a pelvis which their spikes are attached. When threatened, they wiggle their pelvis to raise the spikes. But in the predator-free landlocked lakes, the fish not only don't have the spines, they've lost their pelvis as well. Researchers already knew that a gene called pitxi is linked to the growth of lower limbs in mice. When they deleted the gene in embryos, the mice were not only born with stunted hips and legs, they also died quickly from a variety of ills. Clearly the gene was responsible for multiple functions, so probably the hipless sticklebacks hadn't got rid of the gene altogether.
They found that a short piece of DNA was responsible for turning on the gene at the place where a pelvis would form at the end of the backbone. It turned out that all the landlocked sticklebacks lacked this sequence, but in a variety of different, individual forms - some bits longer or shorter than others.
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And they had done this in less than 12,000 years. So the lesson is that a small change in one tiny spot can lead to dramatic changes in a whole population. If the deletion happened to a fish in the open sea, it would probably be eaten before got a chance to mate, because spikes are a vital part of defence system. But in a predator free lake, it becomes a positive, as less energy devoted to an unnecessary part.
Primate genes plus chimp switches make chimps; primate genes plus human switches make humans.
Lactose tolerance due to a control switch not being turned off in protein which makes enzyme for digesting dairy products.
Every single cell in your body contains more than 2 metres of DNA. Those 2 metres have to be packed up and stored in the cell's nucleus. So, rather than each chromosome being like a long stick of spaghetti, it's more like a tightly packed bowl of ramen noodles in a thick soup of proteins and RNA.
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