Tuesday, 19 April 2016

Is inheritance really all in our genes?

We can inherit a lot from our parents. Hair and eye colour, height. But we can't inherit everything, because some biological traits are acquired during our lifetime. The only way to transmit biological information between generations is the letters of our DNA. But what if its not simple? What if our environment, and our experiences can be passed on to our children and grandchildren? Inheritance is turning out to be much weirder than we think.

Every cell in your body holds an incredible 6 feet (1.8m) of DNA. The same 6 feet of DNA, each holding identical genetic instructions. Yet when skin cells regenerate every day, the new ones somehow "know" to become skin cells, not bone, or muscle. Something beyond DNA influences their destiny. This is what scientists call epigenetics differences in traits that aren't due to changes in the DNA sequence. When it's wrapped up inside the cell, tiny chemical flags on the DNA or the proteins it's coiled around signal the cell to turn certain genes on or off, so they make just the right machinery to do their job. These chemical flags rewritten every day as organisms adapt to new environments, but scientists are seeing something strange: some of these changes can be passed on to the next generation.

Mice fed high-fat diets.. get fat (unsurprisingly) thanks to changes in the chemical flags on their DNA. But female children of these obese mice, even though they were taken away and were raised by normal mothers still ended up 20% fatter than mice from skinny parents. In another example, male mice trained to dear a fruity odour passed sensitivity to this smell on to their children and grandchildren. even thought their offspring had never been exposed to it.

If this sounds a lot like what that guy Lamarck was talking about, well you're not wrong. Before Darwin, may scientists thought acquired traits could be passed on, but natural selection proved that wrong.

But even so, scientists have since seen cases in species from flowers to fruit flies where traits are passed on to children and grandchildren without changing the DNA sequence.

Thre's jus one catch. This shouldn't be possible.

Just hours after an embryo's is conceived, its chemical flags are erased, so all the cell types in the new body can be built from a blank slate. And cells destined to become sperm and eggs get erased a second time. At leasts that's what scientists thought. For epigenetic inheritance to work, some flags must sneak through without being reset. This strange inheritance might even happen in human. During the Dutch famine at the end of WW11, children undernourished in the womb still carried epigenetic changes more than 60 years later. And sicne these changes happen in the womb, they could have a huge effect on our health as adutls.

In Overkalix, Swwden, boys who lived through good harvests had sons and grandsons with higher rates of diabetes and heart disease while boys who lived through winter famines had healthier grandsons-they lived an average 32 years. Strangely, gilrs who lived through swings of feats and famine had granddaughters with higher rates of heart disease.

That is confusing. But human lives aren't easily-controlled lab studies and that's why some scientists doubt this new kind of inhertience.

Epigentic changes can definitely happen between one or two generations, but for a trait to have an effect on evoluation, it had to endure for dozens of generations.

When a baby's developing, the cells that will make a grandchild are aleady present, and can be exposed to do the same environment as the grandmother. That's not inheritance as much as super-duer early exposure. For epigenetic changes to be truly inherited, they have to be rewritten un every generation, we'd have to see them in great-grandchildren and beyond, and that's just not clear yet,

Even so, the vast majority of triats that make us who we are written in our DNA and it's tough to totally rule out genetic changes or other factors even in the cases we've seen. That's the problem with studying complex animals whose lives are the product of thousands of genes in trillions of cells. There's a lot going on here.

But since many of our diseases are linked to stress, diet, or environement, it wouldn't be totally surprising to find out our bodies are affected in ways we didn't know about.

Epigenetics is a young science, and it's reminding us we have a lot of learn about what makes us who we are.

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